umu_test: fix SyntaxError in f-string expression

- It seems we can only use the same outer quotes within f-strings in Python 3.12.

umu/umu_test.py

@@ -1407,7 +1407,7 @@ def test_build_command_nopv(self):
         )
         proton, verb, exe, *_ = [*test_command]
         self.assertEqual(
-            proton, f"{self.env["PROTONPATH"]}/proton", "Expected PROTONPATH"
+            proton, f"{self.env['PROTONPATH']}/proton", "Expected PROTONPATH"
         )
         self.assertEqual(verb, "waitforexitandrun", "Expected PROTON_VERB")
         self.assertEqual(exe, self.env["EXE"], "Expected EXE")