cargo: bump once_cell from 1.7.2 to 1.8.0

Bumps [once_cell](https://github.com/matklad/once_cell) from 1.7.2 to 1.8.0.
- [Release notes](https://github.com/matklad/once_cell/releases)
- [Changelog](https://github.com/matklad/once_cell/blob/master/CHANGELOG.md)
- [Commits](https://github.com/matklad/once_cell/compare/v1.7.2...v1.8.0)

---
updated-dependencies:
- dependency-name: once_cell
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.toml

@@ -47,7 +47,7 @@ native-tls = "0.2.3"
 num_cpus = "1.13.0"
 num-derive = "0.3.0"
 num-traits = "0.2.6"
-once_cell = "1.4.1"
+once_cell = "1.8.0"
 percent-encoding = "2.0"
 pgp = { version = "0.7.0", default-features = false }
 pretty_env_logger = { version = "0.4.0", optional = true }