Ensure firstrun test is immune to system installs

Author: zooba

tests/test_firstrun.py

@@ -102,6 +102,11 @@ def test_check_any_install(fake_config):
 def test_check_latest_install(fake_config, monkeypatch):
     fake_config.default_tag = "1"
     fake_config.default_platform = "-64"
+
+    def _fallbacks(cmd):
+        return [{"install-for": ["0.0-64"]}]
+
+    monkeypatch.setattr(firstrun, "_list_available_fallback_runtimes", _fallbacks)
     assert firstrun.check_latest_install(fake_config) == False
 
     fake_config.installs.append({"tag": "1.0-64"})