automation: use LSB_RELEASE instead of DEBIAN_VERSION

This should be more robust since I believe the minor version can
change mid release.

Differential Revision: https://phab.mercurial-scm.org/D6910

contrib/automation/hgautomation/linux.py

@@ -187,7 +187,7 b' EOF'
 
 sudo apt-key add docker-apt-key
 
-if [ "$DEBIAN_VERSION" = "9.8" ]; then
+if [ "$LSB_RELEASE" = "stretch" ]; then
 cat << EOF | sudo tee -a /etc/apt/sources.list
 # Need backports for clang-format-6.0
 deb http://deb.debian.org/debian stretch-backports main
@@ -271,7 +271,7 b' PACKAGES="\\'
     zip \
     zlib1g-dev"
 
-if [ "$DEBIAN_VERSION" = "9.8" ]; then
+if [ "LSB_RELEASE" = "stretch" ]; then
     PACKAGES="$PACKAGES linux-perf"
 elif [ "$DISTRO" = "Ubuntu" ]; then
     PACKAGES="$PACKAGES linux-tools-common"