Update which requirement from ^4.0 to ^6.0

Updates the requirements on [which](https://github.com/harryfei/which-rs) to permit the latest version.
- [Release notes](https://github.com/harryfei/which-rs/releases)
- [Changelog](https://github.com/harryfei/which-rs/blob/master/CHANGELOG.md)
- [Commits](harryfei/which-rs@4.0.0...6.0.3)

---
updated-dependencies:
- dependency-name: which
  dependency-type: direct:production
...

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

dev-tools/cc-test/Cargo.toml

@@ -12,7 +12,7 @@ test = false
 
 [build-dependencies]
 cc = { path = "../.." }
-which = "^4.0"
+which = "^6.0"
 
 [features]
 parallel = ["cc/parallel"]