enos: support ancient systemd in vault_upgrade (#27960)

Amazon Linux 2 uses an ancient version of Systemd/systemctl so instead
of using -P when determining the unit file we use the less convenient
-p.

Signed-off-by: Ryan Cragun <[email protected]>

enos/modules/vault_upgrade/scripts/maybe-remove-old-unit-file.sh

@@ -18,7 +18,9 @@ fi
 
 # Get the unit file for the vault.service that is running. If it's not in /etc/systemd then it
 # should be a package provided unit file so we don't need to delete anything.
-if ! unit_path=$(systemctl show -P FragmentPath vault 2>&1); then
+#
+# Note that we use -p instead of -P so that we support ancient amzn2 systemctl.
+if ! unit_path=$(systemctl show -p FragmentPath vault | cut -d = -f2 2>&1); then
   echo "Skipped removing unit file because and existing path could not be found: $unit_path"
   exit 0
 fi