ARROW-7880: [CI][R] R sanitizer job is not really working

Closes #6450 from nealrichardson/fix-r-sanitizer and squashes the following commits:

0c953b2 <Neal Richardson> Delete commented code
6af1820 <Neal Richardson> Try setting ARROW_HOME
29a6c85 <Neal Richardson> Try to get r-sanitize job back on track

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>

ci/docker/ubuntu-18.04-r-sanitizer.dockerfile

@@ -76,6 +76,7 @@ RUN echo "MAKEFLAGS=-j$(R --slave -e 'cat(parallel::detectCores())')" >> /usr/lo
 ENV ARROW_DEPENDENCY_SOURCE=SYSTEM \
     ARROW_FLIGHT=OFF \
     ARROW_GANDIVA=OFF \
+    ARROW_HOME=/usr/local \
     ARROW_HDFS=OFF \
     ARROW_INSTALL_NAME_RPATH=OFF \
     ARROW_NO_DEPRECATED_API=ON \

ci/scripts/r_sanitize.sh

@@ -25,6 +25,7 @@ source_dir=${1}/r
 ${R_BIN} CMD INSTALL ${source_dir}
 pushd ${source_dir}/tests
 
+export TEST_R_WITH_ARROW=TRUE
 export UBSAN_OPTIONS="print_stacktrace=1,suppressions=/arrow/r/tools/ubsan.supp"
 ${R_BIN} < testthat.R
 

docker-compose.yml

@@ -784,6 +784,7 @@ services:
         - ${REPO}:amd64-ubuntu-18.04-r-sanitizer
     environment:
       <<: *ccache
+      LIBARROW_BUILD: 'false'
     volumes: *ubuntu-volumes
     command: >
       /bin/bash -c "