build(deps): bump ostree-ext from 0.12.7 to 0.12.8

Bumps [ostree-ext](https://github.com/ostreedev/ostree-rs-ext) from 0.12.7 to 0.12.8.
- [Release notes](https://github.com/ostreedev/ostree-rs-ext/releases)
- [Commits](ostreedev/ostree-rs-ext@ostree-ext-v0.12.7...ostree-ext-v0.12.8)

---
updated-dependencies:
- dependency-name: ostree-ext
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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