CI: Fix incorrect commit hash for signing workflow

When merging the PR for the previous commits, I forgot the note that
stated that it required a merge commit to preserve the pinned commit
hash for the workflow. Thus, when I merged with "Rebase and merge", the
commit hash changed, and the workflow broke.

Update the commit hash to the correct version and fix my mistake.

.github/workflows/push.yaml

@@ -207,7 +207,7 @@ jobs:
 
   sign-windows-build:
     name: Windows Signing ✍️
-    uses: obsproject/obs-studio/.github/workflows/sign-windows.yaml@75ddd93e3ec899ed74f9bd3617c9c26589af2b7e
+    uses: obsproject/obs-studio/.github/workflows/sign-windows.yaml@ffd5879ec95f09e4e551b839f5fe6738058231c1
     if: github.repository_owner == 'obsproject' && github.ref_type == 'tag'
     needs: build-project
     permissions: