libstd: Pass-through PATH in test_override_env

In some build environments (such as chrooted Nix builds), `env` can only
be found in the explicitly-provided PATH, not in default places such as
/bin or /usr/bin. So we need to pass-through PATH when spawning the
`env` sub-process.

Fixes #17617
auto merge of #17667 : wizeman/rust/fix-override-env, r=alexcrichton

In some build environments (such as chrooted Nix builds), `env` can only
be found in the explicitly-provided PATH, not in default places such as
/bin or /usr/bin. So we need to pass-through PATH when spawning the
`env` sub-process.

Fixes #17617