[misc] Skip the protobuf breaking check on branch-creation pushes (#7411)

A push that creates a branch sends the all-zero SHA as `before`, and
bufbuild/buf-action unconditionally builds its default baseline from
it:(src/inputs.ts:86), so `buf breaking` failed cloning a ref that does
not exist. This broke the first run on every new release-* branch, most
recently release-0.78.

Gate `breaking` on github.event.created instead. Nothing is lost: every
commit on a freshly cut release branch already passed the check on main,
and pushes with a real `before` -- plus pull requests, which compare
against their own base -- keep the action's own default baseline, so
stacked PRs are unaffected.

Also drop `build: false`, which is not an input this action accepts and
only produced a warning.

.github/workflows/buf.yml

@@ -27,7 +27,22 @@ jobs:
           push: false
           archive: false
           pr_comment: false
-          build: false
           lint: false
           format: false
-          breaking: true
+          # A push that creates a branch carries no `before` commit, so the
+          # action's default baseline is the all-zero SHA and `buf breaking`
+          # dies cloning it. Skipping costs nothing: every commit on a freshly
+          # cut release branch should have already passed this check on main.
+          breaking: ${{ !github.event.created }}
+          # The alternative is to compare against the default branch instead of
+          # skipping. Not used: buf clones the baseline when the job runs, so a
+          # main that has moved on since the branch was cut reads as protos
+          # deleted on the release branch. Resolving to an empty string on every
+          # other event is what keeps the action's own default in place, which
+          # stacked pull requests need.
+          # breaking_against: >-
+          #   ${{ github.event.created
+          #     && format('{0}#format=git,branch={1}',
+          #               github.event.repository.clone_url,
+          #               github.event.repository.default_branch)
+          #     || '' }}