build(deps): Bump external/fmt from 680db66 to a6c45df

Bumps [external/fmt](https://github.com/fmtlib/fmt) from `680db66` to `a6c45df`.
- [Release notes](https://github.com/fmtlib/fmt/releases)
- [Commits](fmtlib/fmt@680db66...a6c45df)

---
updated-dependencies:
- dependency-name: external/fmt
  dependency-type: direct:production
...

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