[R] Add Raspberry Pi CI build to nightlies

[asfimport]

Now that we support Raspberry Pi OS (#11506 and it might be possible to set up a 32-bit ARM CI which reflects the Rasberry Pi (#11506 (comment) , we could set up a nightly build for this.

Reporter: Nicola Crane / @thisisnic

_{Note: This issue was originally created as ARROW-14455. Please see the migration documentation for further details.}

[pitrou]

Is it useful to keep this open? @thisisnic @jonkeane @nealrichardson

[nealrichardson]

Probably not, if we've gone 3 years without anyone complaining that we're broken on this platform.