ci: only run the expensive tests in the Windows tests for now

Upstream Git does not test their tags with the expensive set of tests,
so a couple of them seem quite broken for now, even so much as hanging
indefinitely.

It is outside of the responsibility of the Git for Windows project to
fix upstream's own tests for platforms other than Windows, so let's not
exercise them.

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

Author: dscho

ci/lib.sh

@@ -319,6 +319,7 @@ export SKIP_DASHED_BUILT_INS=YesPlease
 # enable "expensive" tests for PR events.
 # In order to catch bugs introduced at integration time by mismerges,
 # enable the long tests for pushes to the integration branches as well.
+test -z "$MSYSTEM" ||
 case "$GITHUB_EVENT_NAME,$CI_BRANCH" in
 pull_request,*|push,*next*|push,*master*|push,*main*|push,*maint*)
 	export GIT_TEST_LONG=YesPlease