Fix CircleCI behavior for non-leaf stack PRs (#31088)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31088

Original issue:
https://github.com/pytorch/pytorch/issues/31027

The problem is that for the stacks of PRs for non-leaf PRs circleCI does not set environment variable `CIRCLE_PULL_REQUEST` which is used to filter out some jobs that should run only on `master`.

(Android job for master includes alll 4 abis (x86, x86_64, armeabi-v7a, arm64-v8a)  and gradle build tries to get results from all 4 abis, for PRs we run only x86 build for resources economy. Thats why not filtered master android job fails as abis apart x86 were not scheduled)

env variable `CIRCLE_BRANCH ` is set fine and can be used as a workaround to distinguish that this is PR (published with ghstack).

Test Plan: Imported from OSS

Differential Revision: D18966385

Pulled By: IvanKobzarev

fbshipit-source-id: 644c5ef07fcf2d718b72695da2cc303da8b94ef4
diff --git a/.circleci/scripts/should_run_job.sh b/.circleci/scripts/should_run_job.sh
index 1515014..8dbd7cb 100755
--- a/.circleci/scripts/should_run_job.sh
+++ b/.circleci/scripts/should_run_job.sh
@@ -17,7 +17,7 @@
   echo "You should be running the copy in ~/workspace; SCRIPT_DIR=$SCRIPT_DIR"
   exit 1
 fi
-if [ -n "${CIRCLE_PULL_REQUEST:-}" ]; then
+if [ -n "${CIRCLE_PULL_REQUEST:-}" ] || [[ ${CIRCLE_BRANCH:-} =~ ^gh/.* ]]; then
   if [[ $CIRCLE_BRANCH != "ci-all/"* ]] && [[ $CIRCLE_BRANCH != "nightly" ]] &&  [[ $CIRCLE_BRANCH != "postnightly" ]] ; then
     # Don't swallow "script doesn't exist
     [ -e "$SCRIPT_DIR/should_run_job.py"  ]