arrow

GH-44018: [R] Treat builds on R-universe as like `NOT_CRAN=true` (#44476)

### Rationale for this change

R-universe mimics CRAN's behavior quite well, but unlike CRAN, it does not reject external binary downloads.
So allowing automatic download of libarrow binaries on R-universe may reduce CI run time on R-universe.

### What changes are included in this PR?

Change to attempt to download libarrow binaries on R-universe.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.
* GitHub Issue: #44018

Authored-by: SHIMA Tatsuya <ts1s1andn@gmail.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>