Add flag --print-path to nix print-dev-env, expose getDerivationEnvironment in libcmd

[lisanna-dettwyler]

This new flag prints the store path of the shell environment rather than the contents. Use this new flag to add a garbage collection example in store-delete.md.

Also expose getDerivationEnvironment in libcmd, and in a new stubbed libcmd-c as nix_libcmd_get_derivation_environment. Additional bindings will probably be upstreamed by the cachix team in the future.

@domenkozar you should find this useful for devenv

Motivation

This new flag enables garbage collection of dev shells, which are not directly instantiated.

getDerivationEnvironment is useful for utilities like devenv to compute the same shell environment that nix develop does for a derivation.

Context

#15236
#7239

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[lisanna-dettwyler]

@roberth what are the grammar conventions again for flags?

[roberth]

I'm really not a fan of nix develop, because it's too coupled with nixpkgs and everything.
I think the right direction would be to make shell environment and shell runners part of package expressions instead of continuing to "derive" them from derivations.

Context:

• #7501

[edolstra]

Note that there already is nix develop --profile <profile> for saving a dev shell in a GC-safe way, and nix develop <profile> for re-entering it.

[Ericson2314]

I agree with @roberth that nix_libcmd_get_derivation_environment would disincentivize migrating to #7501. Thank you for volunteering to tackle #7501 --- hopefully if that is done, then then need for all of this goes away, and the annoyance of downstream re-implementing the shell logic (status quo) incentivizes the migration.