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GH-39523: [R] Don't override explicitly set NOT_CRAN=false when on dev version (#39524)

### Rationale for this change

The default linux build used in the lto job should not build with aws/gcs. A change in the build system changed this.

### What changes are included in this PR?

Revert to old behavior by not overriding explicitly set `NOT_CRAN=false`.

### Are these changes tested?

CI

### Are there any user-facing changes?

No
* Closes: #39523

Lead-authored-by: Jacob Wujciak-Jens <jacob@wujciak.de>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>