gha: ci: nightly: Use github.sha to get the last commit reference

As we need to pass down the commit sha to the jobs that will be
triggered from the `schedule` event, we must be careful on what exactly
we're using there.

At first we were using ${{ github.ref }}, but this turns out to be the
**branch name**, rather than the commit hash.  In order to actually get
the commit hash, Let's use ${{ github.sha }} instead, as described by
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#

Fixes: #7247

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

.github/workflows/ci-nightly.yaml

@@ -7,7 +7,7 @@ jobs:
   kata-containers-ci-on-push:
     uses: ./.github/workflows/ci.yaml
     with:
-      commit-hash: ${{ github.ref }}
-      pr-number: ${{ github.ref }}
-      tag: ${{ github.ref }}-nightly
+      commit-hash: ${{ github.sha }}
+      pr-number: ${{ github.sha }}
+      tag: ${{ github.sha }}-nightly
     secrets: inherit