run-tests: disable caching of the wheels when installing test Mercurial

It seems like this command doesn't currently cause the wheels to be cached, but
since I already had to prevent `pip` from caching when building wheels in
1a09563a615c, and also disable it in a test in 30b5dc517ec2, let's just be
explicit that we don't want caching, in case the default behavior changes.

tests/run-tests.py

@@ -3900,6 +3900,7 class TestRunner:
             b"pip",
             b"install",
             wheel_path,
+            b"--no-cache-dir",
             b"--force",
             b"--ignore-installed",
             b"--prefix",