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

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer