pkgs/build-support/trivial-builders: add runCommandLocal

A definition I’ve been copy-pasting everywhere so far, so it’s finally
time to add it to nixpkgs.

I’m using a remote builder for my regular nix builds, so trivial
`runCommand`s which first try a substitution and then copy the inputs
to the builder to run for 0.2s are quite noticable.

If we just always build these, we gain some build time, so let’s make
it easy to switch from remote to local.

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