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ARROW-16334: [Archery][CI] Use build links on nightly report emails instead of branch link

This PR aims to improve the nightly build reporting by changing the link that appears on the report to point to the build instead of the branch. If the task contains a build link it will be shown on the email report. Example of local test:
```bash
$ archery crossbow report --no-fetch nightly-packaging-2022-04-25-0
From: None <None>
To: None
Subject: [NIGHTLY] Arrow Build Report for Job nightly-packaging-2022-04-25-0: 0 failed, 0 pending

Arrow Build Report for Job nightly-packaging-2022-04-25-0

All tasks: https://github.com/ursacomputing/crossbow/branches/all?query=nightly-packaging-2022-04-25-0

Succeeded Tasks:
- conda-osx-clang-py310:
  URL: https://github.com/ursacomputing/crossbow/runs/6156745194
- conda-osx-clang-py38:
  URL: https://github.com/ursacomputing/crossbow/runs/6157874092
- wheel-macos-high-sierra-cp310-amd64:
  URL: https://github.com/ursacomputing/crossbow/runs/6155840346?check_suite_focus=true
```

Closes #13010 from raulcd/ARROW-16334

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>