fix(ci): set datadir env var in serverlist-validate workflow

The serverlist-validate.sh scripts use ${datadir} to cd into the
correct directory, but the workflow never set this variable.
This caused tail to fail with "No such file or directory" as it
ran from the repo root instead of lgsm/data/.

.github/workflows/serverlist-validate.yml

@@ -6,6 +6,9 @@ on:
 permissions:
   contents: read
 
+env:
+  datadir: lgsm/data
+
 jobs:
   serverlist-validate:
     if: github.repository_owner == 'GameServerManagers'