[rule based toolchain] allow env vars in cc_args to be gated by void variables

[cerisier]

cc_args(requires_not_none = ...) works for command-line args gated on types.option(types.void) variables, but fails when the cc_args also sets env.

This blocks toolchains from passing a presence-only signal to wrapper tools through the environment.

Any reason for gating on string/file/directory only ?

[dzbarsky]

@armandomontanez do you happen to recall why this is gated out?

[trybka]

Looks like it was added in #484 by @keith and reviewed by @matts1 who might have more context / memory.

Perhaps just an oversight, though? (We have old-style toolchains with expand_if_available for variables we don't put into the actual env var).

[keith]

yea seems reasonable to support