GitHub: update CI litd branch to testing target

The testing branch in GH CI workflow is switched to `master` from the,
now-merged, development branch 'update-to-lnd-18-4'.

Author: nehhen

.github/workflows/main.yaml

@@ -21,7 +21,7 @@ env:
 
   GO_VERSION: '1.22.6'
 
-  LITD_ITEST_BRANCH: 'update-to-lnd-18-4'
+  LITD_ITEST_BRANCH: 'master'
 
 jobs:
   #######################