use_default_shell_env leads to spurious rebuilds

[mboes]

Describe the problem you are trying to solve.

#2933 introduced a workaround for bazelbuild/bazel#2515. This workaround consists in weakening the hermeticity of the protobuf component of any user's build, by allowing protobuf to depend on LD_LIBRARY_PATH and other environment variables set externally to the build. The problem is that this leads to more frequent rebuilds of all of protobuf. Anytime the PATH or other environment variable changes (which on some systems can happen very frequently), the build cache is invalidated. Since protobuf is a very common dependency very deep in the dependency tree, rebuilding protobuf will affect many users many times. On my team, protobuf rebuilds have become the one thing Bazel is infamous for.

Describe the solution you'd like

Revert #2933. I would argue that setting the right environment variables should be a matter for the toolchain configuration, and that toolchains should otherwise be hermetic. This would lead to far fewer cache invalidations anytime the state of the host environment changes.

Describe alternatives you've considered

An alternative would be to pass through only some variables from the environment, say LD_LIBRARY_PATH, but not PATH. However, this only partially solves the problem above.

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