Coin: fetch repo from github

Git clones from google fail frequently with:
   >> fatal: early EOF
   >> error: index-pack died
Switch to using github mirror for the git-repo repository.

Change-Id: Id0cbf3c59e50869d62880e56251b003490c4c1ee
Reviewed-by: Mikko Gronoff <mikko.gronoff@qt.io>
(cherry picked from commit 61f984c49d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

coin/module_config.yaml

@@ -137,6 +137,7 @@ instructions:
       chmod +x repo
 
       ./repo init \
+          --repo-url https://github.com/theqtcompany/git-repo \
           -u git://code.qt.io/yocto/boot2qt-manifest \
           -b ci \
           -m ${MANIFEST} \