maybe suggest running `cargo update` if there's a previously selected version conflict

I added a new dependency to a project, ran cargo build and got this:

    error: failed to select a version for `libc`
    all possible versions conflict with previously selected versions of `libc`
    required by package `dlopen_derive v0.1.0`
        ... which is depended on by `dlopen v0.1.0`
        ... which is depended on by `ooppkcs11rs v0.0.1 (file:///home/keeler/src/ooppkcs11rs)`
      previously selected package `libc v0.2.23`
        ... which is depended on by `ipc-channel v0.10.0`
        ... which is depended on by `ooppkcs11rs v0.0.1 (file:///home/keeler/src/ooppkcs11rs)`
      possible versions to select: 0.2.40, 0.2.39, 0.2.38, 0.2.37, 0.2.36, 0.2.35, 0.2.34, 0.2.33, 0.2.32, 0.2.31, 0.2.30, 0.2.29 

My assumption was this was because ipc-channel didn't support libc >0.2.23, but there's no such restriction in ipc-channel's Cargo.toml, so this was just confusing. I ended up removing Cargo.lock and rebuilding, and this worked. A colleague later told me cargo update is the real way to fix this.

Reproduce the dependency conflict with cargo build, then trace the Cargo entry point that formats this version-selection error. Check how Cargo.lock and cargo update are handled in the existing user guidance, and verify that the completed change explains the recovery path without misleading users.