ci(linux-asan-ubsan): let's save some time

Every once in a while, the `git-p4` tests flake for reasons outside of
our control. It typically fails with "Connection refused" e.g. here:
https://github.com/git/git/actions/runs/5969707156/job/16196057724

	[...]
	+ git p4 clone --dest=/home/runner/work/git/git/t/trash directory.t9807-git-p4-submit/git //depot
	  Initialized empty Git repository in /home/runner/work/git/git/t/trash directory.t9807-git-p4-submit/git/.git/
	  Perforce client error:
		Connect to server failed; check $P4PORT.
		TCP connect to localhost:9807 failed.
		connect: 127.0.0.1:9807: Connection refused
	  failure accessing depot: could not run p4
	  Importing from //depot into /home/runner/work/git/git/t/trash directory.t9807-git-p4-submit/git
	 [...]

This happens in other jobs, too, but in the `linux-asan-ubsan` job it
hurts the most because that job often takes over a full hour to run,
therefore re-running a failed `linux-asan-ubsan` job is _very_ costly.

The purpose of the `linux-asan-ubsan` job is to exercise the C code of
Git, anyway, and any part of Git's source code that the `git-p4` tests
run and that would benefit from the attention of ASAN/UBSAN are run
better in other tests anyway, as debugging C code run via Python scripts
can get a bit hairy.

In fact, it is not even `git-p4` that is the problem (even if it flakes
often enough to be problematic in the CI builds), but really the part
about Python scripts. So let's just skip any Python parts of the tests
from being run in that job.

For good measure, also skip the Subversion tests because debugging C
code run via Perl scripts is as much fun as debugging C code run via
Python scripts. And it will reduce the time this very expensive job
takes, which is a big benefit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

ci/lib.sh

@@ -280,6 +280,8 @@ linux-leaks)
 	;;
 linux-asan-ubsan)
 	export SANITIZE=address,undefined
+	export NO_SVN_TESTS=LetsSaveSomeTimeBack
+	MAKEFLAGS="$MAKEFLAGS NO_PYTHON=YepItFlakesTooOften"
 	;;
 esac