Stop the PR overlay preview failing stale PRs with SIGPIPE

'git status --short | head' under set -o pipefail: once the PR's
pkgbuilds/ differs from base in more than ten files, head exits, git
takes SIGPIPE and the step fails with 141 before anything builds. Every
PR branched far enough behind master hit it (omadev #423, llmman-bin
#428 and others on 2026-09-28). sed -n '1,10p' prints the same preview
and drains the stream.

.github/workflows/build-pr.yml

@@ -202,7 +202,10 @@ jobs:
           git fetch --no-tags --depth=1 origin "${{ github.event.pull_request.head.sha }}"
           git checkout "${{ github.event.pull_request.head.sha }}" -- pkgbuilds/
           echo "tooling: $(git rev-parse --short HEAD) (base)  packages: ${{ github.event.pull_request.head.sha }} (PR head)"
-          git status --short | head
+          # A preview only. `head` exits after ten lines and, under pipefail,
+          # 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'
       - name: Build ${{ matrix.package }} (${{ matrix.arch }}, ships to ${{ matrix.channels }})
         id: build
         env: