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GH-38811: [R] Actually use fetched cmake on macos (#39453)

### Rationale for this change

We sometimes need to use a more modern cmake, before this change although we downloaded a functioning cmake on macos, we didn't have the correct path for it. 

### What changes are included in this PR?

Resolves #38811 so that cmake is useable when downloaded on macos. This also restores the local source build jobs to be testing that source builds work (which is what the Ci jobs say they are doing). I believe these jobs started using binaries when we overhauled the build system last release.

### Are these changes tested?

Yes, in CI with the local (source) install jobs in crossbow)

### Are there any user-facing changes?

* Closes: #38811

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