Use new cargo-metadata API to check if workspace default members can be read

[sourcefrog]

https://github.com/oli-obk/cargo_metadata/pull/282 adds a new API that we could use to avoid a panic when running with an old cargo toolchain, in https://github.com/sourcefrog/cargo-mutants/blob/196e9d2b31dc33b6cd82416a93bc82aab8aafd07/src/workspace.rs#L100-L103