Bump string_cache from 0.8.6 to 0.8.7

Bumps [string_cache](https://github.com/servo/string-cache) from 0.8.6 to 0.8.7.
- [Release notes](https://github.com/servo/string-cache/releases)
- [Commits](https://github.com/servo/string-cache/commits)

---
updated-dependencies:
- dependency-name: string_cache
  dependency-type: indirect
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]