Add --update-all flag to conda in Gitpod

[mahesh-panchal]

By removing the conda defaults channel from the Dockerfile, the libmamba solver isn't correctly found after installation after package clean up.

$ conda install mamba
Error while loading conda entry point: conda-libmamba-solver (libarchive.so.19: cannot open shared object file: No such file or directory)
Retrieving notices: ...working... done

CondaValueError: You have chosen a non-default solver backend (libmamba) but it was not recognized. Choose one of: classic

This is fixed by using --update-all which updates all package dependencies based on defaults, to use conda-forge. Perhaps defaults is still part of the standard channels on installation.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[jfy133]

Not tested (I assume you have already done so thoroughly :)) but conceptually looks fine