[v1.8.x] Backport #19656 - fix R builds (#19658)

* Attempt to fix R builds.

* Remove obsolete six dependency (#19620)

* Change gcc 8 PPA to ppa:jonathonf/gcc (#19638)

Co-authored-by: Joe Evans <joeev@amazon.com>
Co-authored-by: Ziyi Mu <ziyi.mu@columbia.edu>
Co-authored-by: kpuatamazon <56725192+kpuatamazon@users.noreply.github.com>

Author: 4 people

ci/docker/install/requirements

@@ -31,5 +31,4 @@ pylint==2.3.1  # pylint and astroid need to be aligned
 astroid==2.3.3  # pylint and astroid need to be aligned
 requests<2.19.0,>=2.18.4
 scipy==1.2.1
-six==1.11.0
 setuptools<50

ci/docker/install/ubuntu_gcc8.sh

@@ -17,7 +17,6 @@
 # specific language governing permissions and limitations
 # under the License.
 
-sudo add-apt-repository ppa:jonathonf/gcc-8.0
-sudo add-apt-repository ppa:jonathonf/gcc-7.3
+sudo add-apt-repository ppa:jonathonf/gcc
 sudo apt-get update || true
 sudo apt-get install -y gcc-8 g++-8

ci/docker/install/ubuntu_r.sh

@@ -31,9 +31,13 @@ echo "deb http://cran.rstudio.com/bin/linux/ubuntu trusty/" >> /etc/apt/sources.
 
 apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E298A3A825C0D65DFD57CBB651716619E084DAB9
 
+add-apt-repository ppa:cran/libgit2
+
 apt-get update || true
 apt-get install -y --allow-unauthenticated \
     libcairo2-dev \
+    libgit2-dev \
+    libssh2-1-dev \
     libssl-dev \
     libxml2-dev \
     libxt-dev \