set skipUnless for test that relies on ANDROID_HOME

This job just started failing, but wasn't before:
https://gitlab.com/eighthave/fdroidserver/-/jobs/4060582594

But I look at it, and it looks right that it fails.  So how on earth
was it succeeding before?  Basically the `os.getenv('ANDROID_HOME')`
returns `None` when `ANDROID_HOME` is not set. It is not set in both the
jobs, so how did it not stacktrace before?

tests/common.TestCase

@@ -129,6 +129,7 @@ class CommonTest(unittest.TestCase):
             except fdroidserver.exception.FDroidException:
                 pass
 
+    @unittest.skipUnless(os.getenv('ANDROID_HOME'), "Needs ANDROID_HOME env var")
     def test_find_sdk_tools_cmd(self):
         fdroidserver.common.config = dict()
         # TODO add this once everything works without sdk_path set in config