fix: signoff commit (slsa-framework#767)

Followup to slsa-framework#760

Fix the .github/workflows/update-actions-dist-post-commit.yml workflow
to also signoff commit

# Testing

- [x] Invoked this PR's branch copy of the workflow against slsa-framework#717, and it
did signoff the commit.
-
slsa-framework@9670f76

Signed-off-by: Ramon Petgrave <32398091+ramonpetgrave64@users.noreply.github.com>
Signed-off-by: Ramon Petgrave <ramon.petgrave64@gmail.com>

.github/workflows/update-actions-dist-post-commit.yml

@@ -94,5 +94,5 @@ jobs:
                   git config user.email github-actions@github.com
                   git add .
                   git status
-                  git commit -m "update actions dist"
+                  git commit -s -m "update actions dist"
                   git push