[R] Update rhub/fedora-clang-devel for upstreamed changes

[asfimport]

In ARROW-15857 we fixed the nightly failures on rhub/fedora-clang-devel by a kludge modifying the default makefile, but also upstreamed the fixes (rstudio/sass#104 and r-hub/rhub-linux-builders#60). These upstreams are now both released, so we can remove the kludge from modification of the docker image.

Reporter: Dewey Dunnington / @paleolimbot
Assignee: Dewey Dunnington / @paleolimbot

PRs and other links:

• GitHub Pull Request #12824

_{Note: This issue was originally created as ARROW-16141. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
I tried removing some of this special casing (the parts they took upstream) in ARROW-16752 but had inconsistent results, not sure why (cached docker images?). But anyway I found that we can't yet remove all of the Makevars munging because we edit LDFLAGS and others to be sure that libc++ and only libc++ is used.

[asfimport]

Dewey Dunnington / @paleolimbot:
The sass change isn't on CRAN yet ( https://github.com/rstudio/sass/releases says that the issue was merged but there was a bad merge somewhere and the change isn't actually in the release), so that may be the issue!

[asfimport]

Dewey Dunnington / @paleolimbot:
Issue resolved by pull request 12824
#12824