workflows: checkout nixpkgs into trusted/untrusted directories

By consistently checking out nixpkgs into the same location in every
workflow, it's easier to reason about the different workflows at once.
We also use crystal-clear names to make clear, which checkouts are
considered trusted, because they only contain target-branch-code and
which checkouts are untrusted, because they contain code from the head
branch. By naming the checkout directories trusted/untrusted, it's
obvious at the call-site.

One example of where we likely did the wrong thing is the nixpkgs-vet
workflow: Fetching the toolVersion from the untrusted checkout opens the
door for an injection into the download URL, thus code could be
downloaded from anywhere. This is not a problem, because this workflow
does not run with elevated privileges, but it's a scary oversight
nonetheless.

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