MINOR: Use per-PR concurrency group in Dev PR workflow (#1320)

## What's Changed

The `Dev PR` workflow runs on `pull_request_target`, where `github.ref`
is the base branch (`refs/heads/main`), not the PR ref. Its concurrency
group therefore resolved to `apache/arrow-java-refs/heads/main-Dev PR`
for every PR, and with `cancel-in-progress: true` a new run on one PR
cancelled the run in progress on another.

This leaves a cancelled "Ensure PR format" check on the losing PR, which
shows up as a failure. For example on #1295 the job was cancelled with:

```
Canceling since a higher priority waiting request for apache/arrow-java-refs/heads/main-Dev PR exists
```

This change keys the concurrency group on the PR number, so runs are
only cancelled by newer runs of the same PR.

Note that `pull_request_target` uses the workflow from the base branch,
so this only takes effect once merged.

Author: jbonofre

.github/workflows/dev_pr.yml

@@ -30,7 +30,8 @@ on:
       - review_requested
 
 concurrency:
-  group: ${{ github.repository }}-${{ github.ref }}-${{ github.workflow }}
+  # github.ref is the base branch for pull_request_target, so use the PR number
+  group: ${{ github.repository }}-${{ github.event.pull_request.number }}-${{ github.workflow }}
   cancel-in-progress: true
 
 permissions: