support reading multiple rc files

[aehlig]

Currently, the user-specific rc-file and the rc-file committed into the project are conflicting, in the sense that only one rc-file is used. Instead it would be desirable, that the effects of both are accumulated (as described in https://bazel.build/designs/2016/06/21/environment.html), see, e.g., the discussion in #3516.

[cvcal]

I agree, but this is part of the suggested fix in #4502, can we make that the canonical one? I can change the title to be more inclusive, but it already has the discussion on it, so I'd rather keep it active.

[aehlig]

I agree, but this is part of the suggested fix in #4502, can we make that the canonical one? I can change the title to be more inclusive, but it already has the discussion on it, so I'd rather keep it active.

Sure. Feel free to change the title of #4502 and close this issue as a duplicate.

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