Bump blackfire/php-sdk from 2.3.7 to 2.3.8 in /tools/blackfire

Bumps [blackfire/php-sdk](https://github.com/blackfireio/php-sdk) from 2.3.7 to 2.3.8.
- [Release notes](https://github.com/blackfireio/php-sdk/releases)
- [Changelog](https://github.com/blackfireio/php-sdk/blob/master/CHANGELOG)
- [Commits](blackfireio/php-sdk@v2.3.7...v2.3.8)

---
updated-dependencies:
- dependency-name: blackfire/php-sdk
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>