ci: skip build-and-release on release-please's release PR

[mledour]

Problem

release-please force-updates its release PR (release-please--branches--main) on every push to main. That PR's pull_request event re-triggers build-and-release.yml, so every merge to main now costs two full build runs:

• the push-to-main verification build (wanted), and
• a redundant rebuild of the release PR (not wanted) — it only bumps version.txt + CHANGELOG.md, no code.

Observed after merging #71:
push build + redundant release-PR build.

Fix

Gate the build job:

if: ${{ !startsWith(github.head_ref, 'release-please--') }}

• pull_request from a release-please--* branch → build skipped (no Windows runner consumed).
• push to main / v*.*.* tag → head_ref is empty → unaffected, builds as before.
• The real signed release build still runs on the v*.*.* tag the release PR creates when merged.

main has no branch protection (private repo, free plan), so the skipped check leaves nothing pending on the release PR.

Not changed (intentional)

Merging the release PR still triggers a push-to-main build and the tag build. The push build is mildly redundant with the tag build, but it upholds the workflow's existing "every main commit is green" guarantee and happens only once per release, so it's left as-is. Detecting it would require matching the chore(main): release commit message — too fragile.

🤖 Generated with Claude Code