coverity: cache the Coverity Build Tool

coverity: cache the Coverity Build Tool

It would add a 1GB+ download for every run, better cache it.

This is inspired by the GitHub Action `vapier/coverity-scan-action`,
however, it uses the finer-grained `restore`/`save` method to be able to
cache the Coverity Build Tool even if an unrelated step in the GitHub
workflow fails later on.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>