BUILD: github: run FreeBSD CI on every branch

BUILD: github: run FreeBSD CI on every branch

The FreeBSD workflow was the only workflow in .github/workflows to
restrict itself to the master and next branches. Pull request branches,
merged topic branches and tags were therefore never built or tested on
FreeBSD, unlike macOS and Alpine, which are part of the push-triggered
VTest matrix. As a result changes touching platform specific code could
be merged without any FreeBSD validation.

Drop the branches filter so the workflow triggers on every push, as the
VTest, Windows and Contrib workflows already do. The existing job
condition still limits the run to the haproxy repository, or to a manual
workflow_dispatch, so forks are unaffected.