Add Command::get_resolved_envs

[schneems]

This addition allows an end-user to inspect the environment variables that are visible to the process when it boots.

Discussed in:

• Tracking issue: Introduce a interfaces to expose the current Command captured env var logic #149070 (partially closes)
• ACP: Fix: Introduce an interface to expose the current Command captured env var logic libs-team#194

[rustbot]

r? @Mark-Simulacrum

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[bors]

☔ The latest upstream changes (presumably #149368) made this pull request unmergeable. Please resolve the merge conflicts.

[Mark-Simulacrum]

r=me with nit fixed (and commit squashed for that nit)

I also added a few unresolved questions to the tracking issue but I don't think they're blockers for merging the initial implementation.

View changes since this review

[bors]

☔ The latest upstream changes (presumably #149999) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.