GitHub: temporarily use lnd-19 litd branch for itests

Until we've merged the litd version bump PR for lnd (which in term
requires this PR to be merged), we resolve the circular dependency by
just pointing to that branch to prove the changes in this PR work.

Author: guggero

.github/workflows/main.yaml

@@ -21,7 +21,7 @@ env:
 
   GO_VERSION: '1.23.6'
 
-  LITD_ITEST_BRANCH: 'master'
+  LITD_ITEST_BRANCH: 'lnd-19'
 
 jobs:
   #######################