Can not run outside of cargo project

[foresterre]

The Cargo manifest is resolved to construct the default EnvironmentContext, thus making it impossible to run (with exit code 0) outside a Cargo project. The Cargo project should not be a hard requirement, except for commands which forward directly to Cargo or flags which are meant directly for interaction with Cargo.

See the tests/fixtures/rustc fixture, which should be runnable with cargo msrv find -- rustc hello.rs