Defer fetching of rust-releases index until it's required for a cmd

[foresterre]

e.g. if we now run: cargo msrv list we will fetch the rust-releases index, while this is unnecessary for the list sub command:

    Finished dev [unoptimized + debuginfo] target(s) in 0.19s
     Running `target\debug\cargo-msrv.exe list`
Fetching index
... etc.

[foresterre]

Since https://github.com/foresterre/cargo-msrv/pull/345 we now only fetch the index for commands where we need it; it's however not yet implemented lazilly.