Merge pull request #372 from stan-dev/use-latest-touchstone-pkg-version

Update touchstone GHA to use touchstone@main

Author: VisruthSK

.github/workflows/touchstone-receive.yaml

@@ -41,3 +41,4 @@ jobs:
       - uses: lorenzwalthert/touchstone/actions/receive@main
         with:
           r-version: ${{ matrix.config.r }}
+          touchstone_ref: '@main'