maintainer-scripts: Use /sourceware/snapshot-tmp/gcc as temp directory if possible

> https://gcc.gnu.org/pipermail/gccadmin/2020q4/017037.html
>
> OSError: [Errno 28] No space left on device:
> '/tmp/tmp.Zq3p6D4MxS/gcc/.git/objects/objn31xpefh' ->
> '/tmp/tmp.Zq3p6D4MxS/gcc/.git/objects/db/ffb02a4bcdd4ec04af3db75d86b8cc2e52bdff'
>
> Maybe change the script to use /sourceware/snapshot-tmp/gcc (which has
> rather more space) instead of /tmp?

This patch implements that.

2020-12-17  Jakub Jelinek  <jakub@redhat.com>

	* update_version_git: Put BASEDIR into /sourceware/snapshot-tmp/gcc
	if it exist.