build(deps): bump once_cell from 1.5.2 to 1.6.0

Bumps [once_cell](https://github.com/matklad/once_cell) from 1.5.2 to 1.6.0.
- [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/v1.5.2...v1.6.0)

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

Cargo.lock

@@ -1054,9 +1054,9 @@ dependencies = [
 
 [[package]]
 name = "once_cell"
-version = "1.5.2"
+version = "1.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "13bd41f508810a131401606d54ac32a467c97172d74ba7662562ebba5ad07fa0"
+checksum = "4ad167a2f54e832b82dbe003a046280dceffe5227b5f79e08e363a29638cfddd"
 
 [[package]]
 name = "opaque-debug"