chore: Use full URL for mocks git submodule (#6066)

When working on Invidious forks that are not iv-org/invidious, the git
submodule will try to be fetched from the user fork instead of
`iv-org/mocks`, so it's better to use a full URL instead.

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "mocks"]
 	path = mocks
-	url = ../mocks
+	url = https://github.com/iv-org/mocks.git