arrow

GH-37907: [R] Setting rosetta variable is missing (#37961)



### Rationale for this change

The latest version of `r/R/install-arrow.R`  was not working properly, since it was relying on the `on_rosetta()` function, which is not defined elsewhere. I just fixed the identification of rosetta in the script.

With the current code, the following gives an error

````r
> source("https://raw.githubusercontent.com/apache/arrow/master/r/R/install-arrow.R") 
> install_arrow()
Error in on_rosetta() : could not find function "on_rosetta"
````

### What changes are included in this PR?

It only removed the `on_rosetta()` function, which was not defined elsewhere, and reverted back to the `rosetta` object to identify if rosetta is present or not on a user's system.

### Are these changes tested?

Yes. It was tested with the current code and the proposed PR. The proposed PR works as expected.

### Are there any user-facing changes?

No.

* Closes: #37907

Lead-authored-by: Fernando Mayer <fernandomayer@gmail.com>
Co-authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>