Only force clang, not clang-10

clang-10 isn't available on macOS. Before Bazel 7, this never applied
to macOS since the default toolchain used Xcode, ignoring the value of
`CC` alltogether.

Also move from `--action_env` to `--repo_env`, which doesn't leak into
all actions environment, and don't set `CXX`, which isn't read by Bazel.

.bazelrc

@@ -16,8 +16,7 @@
 common --noenable_bzlmod
 
 # Force the use of Clang for all builds.
-build --action_env=CC=clang-10
-build --action_env=CXX=clang++-10
+build --repo_env=CC=clang
 
 # Needed for abseil-cpp until https://github.com/bazelbuild/bazel/pull/19794 is released.
 build --cxxopt=-std=c++14