Skip packing a PR build when edge already holds the version

A PR can change a package's directory without bumping its version: an
upstream hook, its tests, a README. bin/build then plans nothing, and
the pack step failed on the empty build output, turning the required
result check red (seen on #769). Make the same dry-run check
publish.yml's rebuild job already makes, and skip packing and uploading
when nothing was built. publish.yml finds no artifact for such a merge
and records the package as already published.

.github/workflows/build-pr.yml

@@ -211,11 +211,25 @@ jobs:
           # git's SIGPIPE (141) failed the step for any PR far enough behind
           # master to differ in more files; sed reads the whole stream.
           git status --short | sed -n '1,10p'
+      # A PR can change a package's directory without changing its version:
+      # an upstream hook, its tests, a README. edge already holds that
+      # version, so the planner builds nothing and there is nothing to pack.
+      # The same check publish.yml's rebuild job makes; without it the pack
+      # step failed on the empty output and the required result went red.
       - name: Build ${{ matrix.package }} (${{ matrix.arch }}, ships to ${{ matrix.channels }})
         id: build
         env:
           CONTAINER_ENGINE: docker
-        run: bin/build --mirror edge --arch ${{ matrix.arch }} --package ${{ matrix.package }}
+        run: |
+          set -euo pipefail
+          plan=$(bin/build --dry-run --mirror edge --arch "${{ matrix.arch }}" --package "${{ matrix.package }}" 2>&1 | grep -E '^==> Plan complete' || true)
+          if [[ -n "$plan" && -z "$(sed -E 's/.*would build: *//' <<<"$plan" | tr -d '[:space:]')" ]]; then
+            echo "==> ${{ matrix.package }} (${{ matrix.arch }}): edge already holds this version, nothing to build"
+            echo "built=false" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          bin/build --mirror edge --arch "${{ matrix.arch }}" --package "${{ matrix.package }}"
+          echo "built=true" >> "$GITHUB_OUTPUT"
       # makepkg's check() leaves meson's per-test output in the build tree,
       # never on stdout, so a failing test shows only a summary line in this
       # job log. bin/build bind-mounts $SRC_DIR at /src, so those logs outlive
@@ -243,12 +257,14 @@ jobs:
       # successful build uploads: the artifact's existence is what lets the
       # planner above and publish.yml skip rebuilding this exact tree.
       - name: Pack artifact
+        if: steps.build.outputs.built == 'true'
         id: pack
         run: |
           source helpers/artifact-helpers.sh
           pack_packages build-output/edge/${{ matrix.arch }} packages.tar
           tar -tvf packages.tar
       - name: Upload artifact
+        if: steps.build.outputs.built == 'true'
         uses: actions/upload-artifact@v4
         with:
           name: ${{ matrix.package }}-${{ matrix.arch }}-${{ steps.tree.outputs.hash }}