CI: add concurrency support to zfs-arm

[chrislongros]

Motivation and Context

The zfs-arm workflow was the only build/test workflow without a concurrency block. zfs-qemu, zfs-qemu-packages, zloop, codeql, smatch, and checkstyle all define one; zfs-arm did not, so superseded runs on a branch were not cancelled when a new commit was pushed.

Description

Add the same concurrency block the other workflows use, so a new push to a branch cancels the in-progress ARM run instead of leaving both to finish:

concurrency:
  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
  cancel-in-progress: true

How Has This Been Tested?

Workflow-only change. CI will tell.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes. (N/A — CI config)
• I have run the ZFS Test Suite with this change applied. (N/A — CI config)
• All commit messages are properly formatted and contain Signed-off-by.