ci: Set --nocleanup for Windows functional tests

This avoids intermittent issues in the CI Task when a test passes
successfully. For example:

PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'C:\\Users\\ContainerAdministrator\\AppData\\Local\\Temp\\test_runner_₿_🏃_20210920_072037\\feature_versionbits_warning_89\\alert.txt'

.cirrus.yml

@@ -148,7 +148,7 @@ task:
     - python test\util\rpcauth-test.py
   functional_tests_script:
     # TODO enable '--extended' and drop '--exclude'.
-    - python test\functional\test_runner.py --ci --quiet --combinedlogslen=4000 --jobs=4 --timeout-factor=8 --exclude %EXCLUDE_TESTS% --failfast
+    - python test\functional\test_runner.py --nocleanup --ci --quiet --combinedlogslen=4000 --jobs=4 --timeout-factor=8 --exclude %EXCLUDE_TESTS% --failfast
 
 task:
   name: 'ARM [unit tests, no functional tests] [bullseye]'