ci: run the unit-test gate on PRs to master

[cplieger]

The Unit Tests workflow filters pull_request to branches: [main], but the repository's default branch is master, so the gate has never run on any PR. That's how #228 merged a notifykit bump that doesn't compile (details in #242). One-line fix: point the filter at master.

Two things worth knowing before merging:

• This PR itself won't trigger the workflow: the paths filter only matches internal/**, main.go, go.mod, and go.sum, and this change touches none of them. The first PR that touches Go code after this merges will run the gate.
• Because master currently doesn't compile (master does not compile; the PR test gate never runs #242), that first Go-touching PR will be red until internal/notify is adapted to the new notifykit API. That's the gate working as intended, but if you'd rather not have a red window, fixing master does not compile; the PR test gate never runs #242's compile break first and merging this right after avoids it.

The workflow_call and workflow_dispatch triggers are untouched.

Checklist

• The PR has a meaningful title (changelog-ready).
• PR contains a single logical change.
• Documentation: nothing to update.
• Labels: I can't set labels on this repo; change (or bug) fits.
• Linked related issues/PRs: fixes the CI half of master does not compile; the PR test gate never runs #242.