ActionBuilder: Define compiler-specific defaults for `RECC_ENV_TO_READ`

Jürg Billeterrequested to merge
juerg/env into master

Environment variables may affect the compiler output, which means that relevant environment variables have to be included in the generated Command proto to avoid incorrect cache hits and ensure expected compiler output with remote execution. As the relevant set of environment variables depend on the compiler, configuring this via RECC_ENV_TO_READ is not trivial, especially when working with multiple compilers.

This branch defines compiler-specific defaults for RECC_ENV_TO_READ to make it easier to use recc correctly.

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