snap: fix generated postrefreshhook script (#9994)

Fixes #9990

If the python oneliner to check certbot's version succeeded, exit_code
would never be set, which would cause our exit_code check to fail. Use
a check that handles unset exit_code

tools/snap/generate_dnsplugins_postrefreshhook.sh

@@ -25,7 +25,7 @@ cb_required=\$(grep -oP "version = '\K.*(?=')" \$SNAP/setup.py)
 
 \$SNAP/bin/python3 -c "import sys; from packaging import version; sys.exit(1) if\
   version.parse('\$cb_installed') < version.parse('\$cb_required') else sys.exit(0)" || exit_code=\$?
-if [ "\$exit_code" -eq 1 ]; then
+if [ "\$exit_code" = "1" ]; then
   echo "Certbot is version \$cb_installed but needs to be at least \$cb_required before" \\
     "this plugin can be updated; will try again on next refresh."
   exit 1