test if TMPDIR is accessible before using

test if TMPDIR is accessible before using

Private temporary directories as created by e.g. libpam-tmpdir are nice,
but they are also very effective in preventing our priviledge dropping
to work as TMPDIR will be set to a directory only root has access to, so
working with it as _apt will fail. We circumvent this by extending our
check for a usable TMPDIR setting by checking access rights.

Closes: 765951