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GH-38570: [R] Ensure that test-nix-libs is warning free (#38571)

### Rationale for this change

Although we stop / fail the CI job for warnings elsewhere, we don't seem to be doing that for our buildsystem tests. Let's change that so warnings + other output doesn't creep in.

- [x] Prevent `*** Failed to find latest nightly for 8.0.0.9000` from showing up when the file is sourced under test
- [x] fix `Warning ('test-nixlibs.R:140:3'): select_binary() with test program 'x' is NULL so the result will be NULL`
- [x] update version strings to be characters to avoid r-devel warnings

### What changes are included in this PR?

Added `stop_on_warning` to the test call, and fixes to make that pass

### Are these changes tested?

Yes, they are all part of the test suite 

### Are there any user-facing changes?

No.
* Closes: #38570

Authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>