Mirror the tag object, not the commit, in vtag.yml

The v-prefixed mirror was created from github.sha, so for an annotated or
signed tag it would point at the commit and drop the signature: "git
verify-tag v3.1.3" fails with "cannot verify a non-tag object of type
commit" while "git verify-tag 3.1.3" succeeds. Resolve refs/tags/<tag>
and mirror whatever object it points at instead, which keeps the current
behaviour for lightweight tags. Also move the workflow expressions into
env instead of interpolating them into the shell command.

.github/workflows/vtag.yml

@@ -23,10 +23,22 @@ jobs:
       - name: Create the v-prefixed tag
         env:
           GH_TOKEN: ${{ github.token }}
+          REPO: ${{ github.repository }}
+          TAG: ${{ github.ref_name }}
         run: |
-          if gh api "repos/${{ github.repository }}/git/ref/tags/v${{ github.ref_name }}" >/dev/null 2>&1; then
-            echo "Tag v${{ github.ref_name }} already exists, nothing to do."
+          if gh api "repos/$REPO/git/ref/tags/v$TAG" >/dev/null 2>&1; then
+            echo "Tag v$TAG already exists, nothing to do."
             exit 0
           fi
-          gh api "repos/${{ github.repository }}/git/refs" -f ref="refs/tags/v${{ github.ref_name }}" -f sha="${{ github.sha }}"
-          echo "Created tag v${{ github.ref_name }} -> ${{ github.sha }}"
+          # Mirror the object the pushed tag actually points at: the commit
+          # for a lightweight tag, the tag object itself for an annotated or
+          # signed one. Pointing the mirror at the commit would strip the
+          # signature, so "git verify-tag v3.1.3" would fail while
+          # "git verify-tag 3.1.3" succeeds.
+          sha="$(gh api "repos/$REPO/git/ref/tags/$TAG" --jq .object.sha)"
+          if [ -z "$sha" ] || [ "$sha" = "null" ]; then
+            echo "Could not resolve refs/tags/$TAG"
+            exit 1
+          fi
+          gh api "repos/$REPO/git/refs" -f ref="refs/tags/v$TAG" -f sha="$sha"
+          echo "Created tag v$TAG -> $sha"