arrow

GH-38430: [R] Add test + fix corner cases after nixlibs.R refactor (#38534)


### Rationale for this change

A few rough edges exist after https://github.com/apache/arrow/pull/38236 including:

- When zero or 1 nightly with the matching major version exist, detection of the latest nightly might fail
- At least one CI job is pulling nightlies instead of using the version from the current commit

### What changes are included in this PR?

- Clean up `find_latest_nightly()` + add test
- Ensure all CI jobs are not using `find_latest_nightly()`

### Are these changes tested?

Yes (test added)

### Are there any user-facing changes?

No
* Closes: #38430

Lead-authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Co-authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>