ignore for _apt inaccessible TMPDIR in pkgAcqChangelog

ignore for _apt inaccessible TMPDIR in pkgAcqChangelog

Using libpam-tmpdir caused us to create our download tmp directory in
root's private tmp before changing to _apt, which wouldn't have access
to it.

By extending our GetTempDir method with an optional wrapper changing the
effective user, we can test if a given user can access the directory and
ignore TMPDIR if not instead of ignoring TMPDIR completely.

Closes: 797270