Output complete repro command when failing to generate rustdoc JSON.

[obi1kenobi]

Resolves #570 by making it more likely that users are able to resolve the build error on their own, instead of being confused about why cargo-semver-checks is failing to build their project.

For example, for #570 we now output:

error: failed to build rustdoc for crate boa_icu_provider v0.17.3
note: this is usually due to a compilation error in the crate,
      and is unlikely to be a bug in cargo-semver-checks
note: the following command can be used to reproduce the compilation error:
      cargo new --lib example &&
          cd example &&
          echo '[workspace]' >> Cargo.toml &&
          cargo add --path /tmp/boa/boa_icu_provider --no-default-features --features bin,default,full,std &&
          cargo check

which I've checked correctly reproduces the build issue. The previous advice to run cargo build surprisingly did not reproduce the problem, which is why I'm replacing it with this new command.