Honor source-repo-ref when checking out for release dispatch (DataDog#23836)

The release-trigger workflow accepts a source-repo-ref input, but the
reusable release-dispatch workflow's actions/checkout step did not pass
a ref, so it always checked out github.sha (the workflow's commit).

As a result, ddev release tag and the validation steps ran against the
wrong commit when a manual release was dispatched against a non-HEAD
ref. Pass inputs.source-repo-ref (with github.sha as the fallback) to
actions/checkout so the working tree matches the requested ref.

Author: dkirov-dd

.github/workflows/release-dispatch.yml

@@ -58,6 +58,7 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ inputs.source-repo-ref || github.sha }}
           fetch-depth: 0 # ddev needs full tag history
 
       - name: Checkout integrations-core actions