Disable the API breakage check in GitHub Actions. (#1311)

Disable the API breakage check in GitHub Actions. We are not targetting
the 6.1 toolchain, so we aren't worried about API breakage when built
against it.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

.github/workflows/pull_request.yml

@@ -27,3 +27,4 @@ jobs:
       docs_check_enabled: false
       unacceptable_language_check_enabled: false
       format_check_enabled: false
+      api_breakage_check_enabled: false