Auto merge of #38856 - zackw:process-envs, r=aturon

Add std::process::Command::envs()

`Command::envs()` adds a vector of key-value pairs to the child
process environment all at once.  Suggested in #38526.

This is not fully baked and frankly I'm not sure it even _works_, but I need some help finishing it up, and this is the simplest way to show you what I've got.  The problems I know exist and don't know how to solve, from most to least important, are:

* [ ] I don't know if the type signature of the new function is correct.
* [x] The new test might not be getting run.  I didn't see it go by in the output of `x.py test src/libstd --stage 1`.
* [x] The tidy check says ``process.rs:402: different `since` than before`` which I don't know what it means.

r? @brson

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