Use git.h.o copy of arcanist-external-json-linter

It shouldn't have been possible to reference an external
Git submodule not hosted on git.haskell.org as we can't otherwise
ensure gitlink integrity. But it turns out the
validation hook scripts in place didn't reject
020375d1, so here we are...

This commit changes ghc.git to use our own fork/copy of
https://github.com/bgamari/arcanist-external-json-linter
hosted on git.haskell.org

.gitmodules

@@ -117,4 +117,4 @@
 	ignore = none
 [submodule ".arc-linters/arcanist-external-json-linter"]
 	path = .arc-linters/arcanist-external-json-linter
-	url = https://github.com/bgamari/arcanist-external-json-linter.git
+	url = ../arcanist-external-json-linter.git