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.
