MinGW's `env::vars_os` and `env::var_os` disagree

[tbu-]

See also #29910, #29297 (comment) and the relevant test file src/test/run-pass/env-vars.rs: permanent link, current version.

[alexcrichton]

After some investigation, this is accounted for by interpreting a leading = sign differently.

[tbu-]

I guess with #29297 merged this could be closed?

[alexcrichton]

I think so, yeah, thanks @tbu-!