.github/workflows: fix typo in XDG_CACHE_HOME

This appears to be one of the contributors to this CI target regularly
entering a bad state with a partially written toolchain.

Updates #self

Signed-off-by: James Tucker <james@tailscale.com>

.github/workflows/test.yml

@@ -208,7 +208,7 @@ jobs:
       env:
         HOME: "/tmp"
         TMPDIR: "/tmp"
-        XDB_CACHE_HOME: "/var/lib/ghrunner/cache"
+        XDG_CACHE_HOME: "/var/lib/ghrunner/cache"
 
   race-build:
     runs-on: ubuntu-22.04