Add GITEA_PR_INDEX env variable to githooks (#35938)

`GITEA_PR_ID` is already part of the env variables available in the
githooks, but it contains a database ID instead of commonly used index
that is part of `owner/repo!index`

services/pull/merge.go

@@ -403,6 +403,7 @@ func doMergeAndPush(ctx context.Context, pr *issues_model.PullRequest, doer *use
 		pr.BaseRepo,
 		pr.BaseRepo.Name,
 		pr.ID,
+		pr.Index,
 	)
 
 	mergeCtx.env = append(mergeCtx.env, repo_module.EnvPushTrigger+"="+string(pushTrigger))