qa/standalone/scrub: s/$(pgid)/${pgid}/

qa/standalone/scrub: s/$(pgid)/${pgid}/

to address the test failures like
```
2020-04-07T15:44:58.693 INFO:tasks.workunit.client.0.smithi049.stderr:/home/ubuntu/cephtest/clone.client.0/qa/standalone/scrub/osd-scrub-repair.sh:498: TEST_auto_repair_bluestore_failed:  ceph pg dump
pgs
2020-04-07T15:44:58.694 INFO:tasks.workunit.client.0.smithi049.stderr://home/ubuntu/cephtest/clone.client.0/qa/standalone/scrub/osd-scrub-repair.sh:498: TEST_auto_repair_bluestore_failed:  pgid
2020-04-07T15:44:58.694 INFO:tasks.workunit.client.0.smithi049.stderr:/home/ubuntu/cephtest/clone.client.0/qa/standalone/scrub/osd-scrub-repair.sh: line 498: pgid: command not found
```

Signed-off-by: Kefu Chai <kchai@redhat.com>