lib: retry git fetch a few times

The instance hosting the forgejo repository may be down or very slow
while the build tries to fetch from it, retry a few times instead of
giving up immediately.

A total retry time of 3 * 60 seconds is probably sufficient since
Forgejo startup time is lower than this, even on large instances.

lib/build.sh

@@ -25,6 +25,11 @@ fi
 if ! [[ "$ref" =~ ^refs/ ]] ; then
     ref=refs/heads/$ref
 fi
+for retry in 1 2 3 ; do
+    timeout 15m git fetch --update-head-ok origin +$ref:$ref && break
+    echo "Retry git fetch in 60 seconds"
+    sleep 60
+done
 git fetch --update-head-ok origin +$ref:$ref
 git switch --force-create $v $ref