conda build crashes on non-BSD systems if bash is not installed

[dHannasch]

Actual Behavior

If bash is not installed (as it is not in, among others, the official Anaconda docker images), then conda-build crashes.

$ conda build .
Traceback (most recent call last):
  File "/opt/conda/envs/test-env/bin/conda-build", line 11, in <module>
    sys.exit(main())
  File "/opt/conda/envs/test-env/lib/python3.7/site-packages/conda_build/cli/main_build.py", line 469, in main
    execute(sys.argv[1:])
  File "/opt/conda/envs/test-env/lib/python3.7/site-packages/conda_build/cli/main_build.py", line 460, in execute
    verify=args.verify, variants=args.variants)
  File "/opt/conda/envs/test-env/lib/python3.7/site-packages/conda_build/api.py", line 209, in build
    notest=notest, need_source_download=need_source_download, variants=variants)
  File "/opt/conda/envs/test-env/lib/python3.7/site-packages/conda_build/build.py", line 2344, in build_tree
    notest=notest,
  File "/opt/conda/envs/test-env/lib/python3.7/site-packages/conda_build/build.py", line 1492, in build
    cwd=src_dir, stats=build_stats)
  File "/opt/conda/envs/test-env/lib/python3.7/site-packages/conda_build/utils.py", line 398, in check_call_env
    return _func_defaulting_env_to_os_environ('call', *popenargs, **kwargs)
  File "/opt/conda/envs/test-env/lib/python3.7/site-packages/conda_build/utils.py", line 373, in _func_defaulting_env_to_os_environ
    proc = PopenWrapper(_args, **kwargs)
  File "/opt/conda/envs/test-env/lib/python3.7/site-packages/conda_build/utils.py", line 258, in __init__
    self.out, self.err = self._execute(*args, **kwargs)
  File "/opt/conda/envs/test-env/lib/python3.7/site-packages/conda_build/utils.py", line 284, in _execute
    _popen = psutil.Popen(*args, **kwargs) if psutil else subprocess.Popen(*args, **kwargs)
  File "/opt/conda/envs/test-env/lib/python3.7/site-packages/psutil/__init__.py", line 1430, in __init__
    self.__subproc = subprocess.Popen(*args, **kwargs)
  File "/opt/conda/envs/test-env/lib/python3.7/subprocess.py", line 800, in __init__
    restore_signals, start_new_session)
  File "/opt/conda/envs/test-env/lib/python3.7/subprocess.py", line 1551, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: '/bin/bash': '/bin/bash'

@xhochy traced this back to bash being hardcoded into build.py: https://github.com/conda/conda-build/blob/master/conda_build/build.py#L82
There's some chatter on the Docker images,
anaconda/docker-images#120 (comment)
anaconda/docker-images#160
but there doesn't seem to be a conda-build issue for this yet, and it seems like conda-build really ought to not crash regardless of the image.
@mingwandroid suggested that conda-build could depend on bash: anaconda/docker-images#159 (comment)
It also seems worth asking whether conda-build even really needs bash --- if conda-build can get by with /bin/sh on BSD systems, could it do the same on, for example, Alpine systems?
https://github.com/conda/conda-build/blob/master/conda_build/build.py#L79

(Obviously, apk add bash works as a workaround, and presumably installing bash will also solve the problem on non-Alpine systems.)

Expected Behavior

conda build should not crash.

Steps to Reproduce

Try to run conda build on a non-BSD image that does not have bash. I'm partial to frolvlad/alpine-miniconda3 since that's what taught me how to make conda activate work, but as mentioned, continuumio/miniconda3 also does not have bash, so conda-build does not work. anaconda/docker-images#162 (comment)

Output of conda info

$ conda info

     active environment : test-env
    active env location : /opt/conda/envs/test-env
            shell level : 1
       user config file : /root/.condarc
 populated config files : /root/.condarc
          conda version : 4.7.12
    conda-build version : not installed
         python version : 3.7.4.final.0
       virtual packages : 
       base environment : /opt/conda  (writable)
           channel URLs : https://repo.anaconda.com/pkgs/main/linux-64
                          https://repo.anaconda.com/pkgs/main/noarch
                          https://repo.anaconda.com/pkgs/r/linux-64
                          https://repo.anaconda.com/pkgs/r/noarch
          package cache : /opt/conda/pkgs
                          /root/.conda/pkgs
       envs directories : /opt/conda/envs
                          /root/.conda/envs
               platform : linux-64
             user-agent : conda/4.7.12 requests/2.22.0 CPython/3.7.4 Linux/3.10.0-957.5.1.el7.x86_64 alpine/3.10.2 glibc/2.30
                UID:GID : 0:0
             netrc file : None
           offline mode : False

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