Support non-UTF-8 in environment variable dependency tracking

[madsmtm]

ParseSess::env_depinfo stores Symbol instead of OsStr.

While we're at it, might make sense to actually not store the environment variable value at all, and instead fetch it inside write_out_deps? (which assumes that the variable won't change during execution of the compiler, but it probably won't).

See also #130883 (comment).

[bjorn3]

Cargo expects the dep-info file to be valid UTF-8 and parse_rustc_dep_info returns an error when it isn't. As the env var value is included in the dep-info file, this implies that the env var value itself has to be UTF-8 too.

[petrochenkov]

In the original PR introducing the env depinfo we discussed support for non UTF-8, and the outcome was to postpone it until there's a sufficient motivation, it can always be done with some new directive, # env-dep-escaped or something (#71858 (comment)).

#130883 (comment) merely suggests moving the recovery point for non-UTF-8 env vars to fn write_out_deps.

[madsmtm]

#130883 (comment) merely suggests moving the recovery point for non-UTF-8 env vars to fn write_out_deps.

Sorry, I misunderstood, then. Still, I think keeping it separate from that PR is valuable