Move supported distros to variable

Improve redability of the distro check using a variable to
store supported distributions.

Change-Id: Iffa1619065358d459bd04523b7eeae9049f8f2f0
diff --git a/stack.sh b/stack.sh
index 3b2a331..bb4dfa2 100755
--- a/stack.sh
+++ b/stack.sh
@@ -221,7 +221,9 @@
 
 # Warn users who aren't on an explicitly supported distro, but allow them to
 # override check and attempt installation with ``FORCE=yes ./stack``
-if [[ ! ${DISTRO} =~ (bionic|focal|f31|f32|opensuse-15.2|opensuse-tumbleweed|rhel8) ]]; then
+SUPPORTED_DISTROS="bionic|focal|f31|f32|opensuse-15.2|opensuse-tumbleweed|rhel8"
+
+if [[ ! ${DISTRO} =~ $SUPPORTED_DISTROS ]]; then
     echo "WARNING: this script has not been tested on $DISTRO"
     if [[ "$FORCE" != "yes" ]]; then
         die $LINENO "If you wish to run this script anyway run with FORCE=yes"