fix: duplicate key violates unique constraint in concurrent debian package uploads (#11776)

Fixes #11438.

Whenever a "unique constraint violation" error is encountered by package mutation, detect if a `xorm.ErrUniqueConstraintViolation` error occurs.  If it does, retry the entire transaction.

## Checklist

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### Tests for Go changes

(can be removed for JavaScript changes)

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
  - [ ] `make pr-go` before pushing

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [ ] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11776
Reviewed-by: Andreas Ahlenstorf <aahlenst@noreply.codeberg.org>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>

.deadcode-out

@@ -19,7 +19,6 @@ forgejo.org/models/auth
 forgejo.org/models/db
 	TruncateBeans
 	TruncateBeansCascade
-	InTransaction
 	DumpTables
 	GetTableNames
 	extendBeansForCascade