automation: make clean between builds

automation: make clean between builds

Currently randconfig tests are more likely to fail than to succeed
because of a bug in xen's build system: symbols-dummy.o's dependency
is wrong, which causes it to not get rebuild between runs, which
eventually causes linking to fail. There may also be other corner
cases we haven't discovered.

The fix is not straightforward. For now, make sure the tree is cleaned
properly between builds so we don't see random failures in Gitlab CI.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Doug Goldstein <cardoe@cardoe.com>