Fix Internal Tests paths filter checkout (#5295)

## What changed

Adds an explicit checkout step before `dorny/paths-filter` in the
Internal Tests workflow.

## Why

`dorny/paths-filter@v3` needs a git working tree for `push` events. The
Internal Tests workflow ran it before any checkout, so every `push` run
on `master` failed immediately in `Detect non-docs changes` with:

```text
fatal: not a git repository (or any of the parent directories): .git
```

This only showed up consistently on `master` because those runs are
`push` events. On `pull_request` events, `dorny/paths-filter` can use
the GitHub pull request files API with the PR number, so it does not
need a local checkout for the same file detection path.

Adding checkout gives the action a repository when it handles `push`
events, while leaving PR behavior unchanged.

## Testing

- `git diff --check`
- PR #5295 `Internal Tests` job completed `Checkout` and `Detect
non-docs changes` successfully, then moved on to private dispatch/wait.

---------

Signed-off-by: Zeke Foppa <196249+bfops@users.noreply.github.com>
Co-authored-by: clockwork-labs-bot <clockwork-labs-bot@users.noreply.github.com>
Co-authored-by: Zeke Foppa <196249+bfops@users.noreply.github.com>

Author: clockwork-labs-bot

.github/workflows/internal-tests.yml

@@ -30,6 +30,10 @@ jobs:
       TARGET_OWNER: clockworklabs
       TARGET_REPO: SpacetimeDBPrivate
     steps:
+      # Required for the path filter check below on non-PR commits (e.g. `master`)
+      - name: Checkout
+        uses: actions/checkout@v4
+
       # Skip the private dispatch entirely when only `docs/` is touched. The job
       # itself still completes successfully so required-status-check gating is
       # satisfied without spending private-runner time on a docs-only change.