fix(hook): instance admins wrongly restricted by permissions checks

This exception existed for both instance admins and repo admins
before ApplyToAdmins was introduced in
79b7089360.

It should have been kept for instance admins only because they are not
subject to permission checks.

(cherry picked from commit 05f0007437)

routers/private/hook_pre_receive.go

@@ -398,6 +398,11 @@ func preReceiveBranch(ctx *preReceiveContext, oldCommitID, newCommitID string, r
 			return
 		}
 
+		// If we're an admin for the instance, we can ignore checks
+		if ctx.user.IsAdmin {
+			return
+		}
+
 		// It's not allowed t overwrite protected files. Unless if the user is an
 		// admin and the protected branch rule doesn't apply to admins.
 		if changedProtectedfiles && (!ctx.user.IsAdmin || protectBranch.ApplyToAdmins) {