chore(linux): Fix GHA triggering

When we build a PR on TC we use `pull/ID/head` for the build, not
`pull/ID/merge`, i.e. the tip of the feature branch and not a merge
with the base branch. This means that during the build `pull/ID/merge`
is unknown. This change modifies the trigger to use `pull/ID/head` for
GHA builds as well.

It's still a mystery why `pull/ID/merge` was known to builds in most
cases but failed for some PRs.