Fix CI failures

[yutannihilation]

Summary of the changes

• Skip some tests if quantreg package is not available
• Replace macOS + R-devel runner and with Ubuntu-16.04 + R-devel runner
• Install binary packages only on macOS
• Remove the step to add libgit PPA

Details of the problems

quantreg on R < 3.6

As of version 5.61 (released on July 7, 2020), quantreg package imports conquer package, which requires R >= 3.6. So, quantreg now virtually requires R >= 3.6.

Diff: cran/quantreg@16f4941#diff-35ba4a2677442e210c23a00a5601aba3R31

macOS

R-devel

It fails to setup R for an unknown error. Now that r-lib/actions provide R-devel also to Linux runners, let's use ubuntu-16.04 for R-devel. I feel the toolchain of macOS is too franky, and as ggplot2 is not a package that needs compilation, I feel the macOS + R-devel is not worth keeping.

r-lib/actions#49 (comment)

R 4.0

brew install gdal fails with the following error. I set the pkgType option on macOS to avoid building packages at all.

==> Installing gdal dependency: gcc
==> Pouring gcc-10.1.0.catalina.bottle.tar.gz
##[error]Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
Could not symlink bin/gfortran
Target /usr/local/bin/gfortran
already exists. You may want to remove it:
  rm '/usr/local/bin/gfortran'

To force the link and overwrite all conflicting files:
  brew link --overwrite gcc

To list all files that would be deleted:
  brew link --overwrite --dry-run gcc

Possible conflicting files are:
/usr/local/bin/gfortran -> /usr/local/gfortran/bin/gfortran

libgit2 PPA

(This is not about failures, just for cleanup)

As r-lib/actions#97 is already merged, we can just remove this from the YAML.

[yutannihilation]

For brew gdal install failure, I asked on RStudio Community.

https://community.rstudio.com/t/brew-install-gdal-errors-on-github-actions-ci-macos-runners/74336?u=yutannihilation

[yutannihilation]

For R-devel failure, we can watch on this.
r-lib/actions#140 (comment)

[yutannihilation]

I think this is ready now (but not in a hurry to be reviewed).

[karawoo]

LGTM

[yutannihilation]

Thanks!