chore(test runner): document that --only-changed on CI needs history (#32461)

Closes https://github.com/microsoft/playwright/issues/32452

`--only-changed=$GITHUB_BASE_REF` needs the base ref available locally
to work properly. `fetch-depth: 0` does that, see
https://github.com/actions/checkout?tab=readme-ov-file#fetch-all-history-for-all-tags-and-branches.

---------

Signed-off-by: Simon Knott <info@simonknott.de>
Co-authored-by: Dmitry Gozman <dgozman@gmail.com>

docs/src/ci.md

@@ -432,6 +432,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
+      with:
+        # Force a non-shallow checkout, so that we can reference $GITHUB_BASE_REF.
+        # See https://github.com/actions/checkout for more details.
+        fetch-depth: 0
     - uses: actions/setup-node@v4
       with:
         node-version: 18
@@ -1044,4 +1048,4 @@ xvfb-run mvn test
 ```
 ```bash csharp
 xvfb-run dotnet test
-```
+```