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ARROW-14992: [R] Installation can't use prebuilt Arrow binaries on Pop! OS

This PR makes a few small changes to help users (and maintainers responding to install issues) troubleshoot installing when there isn't a prebuilt binary for a given platform/Arrow version. It's difficult to test this since there isn't a prebuilt Arrow binary for master but perhaps there is a way to do this that I'm not aware of. I verified that it downloads a prebuilt binary on a Pop! OS Docker image. The install still fails because there's no prebuilt binary for the dev version (but at least it's in the error message).

``` bash
# docker pull hugojosefson/popos
# docker run --rm -it hugojosefson/popos
apt-get update && apt-get install -y r-base
R -e "install.packages(c('remotes', 'bit', 'magrittr', 'ellipsis', 'glue', 'assertthat', 'bit64', 'purrr', 'R6', 'rlang', 'tidyselect', 'vctrs'))"
LIBARROW_BINARY=ubuntu-18.04 NOT_CRAN=true R -e "remotes::install_github('apache/arrow/r')"
# *** Proceeding without C++ dependencies

LIBARROW_BINARY=ubuntu-18.04 NOT_CRAN=true R -e "remotes::install_github('paleolimbot/arrow/r@r-nixlibs')"
# *** No Arrow binary found for version 6.0.1.9000 on ubuntu-18.04
# *** Proceeding without C++ dependencies
```

Closes #11870 from paleolimbot/r-nixlibs

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>