Use `--locked` flag for Cargo install

[allan2]

Omitting the --locked flag can cause installation to fail when dependencies have broken updates.

For example, trying to install the 0.100.0 release using cargo install nu fails for me due to an issue with uutils.

I suggest adding the --locked flag to all cargo install references in the docs. I'd be happy to open a PR.

[fdncred]

There's only been a couple times over the last 5 years that --locked has been required. However, I don't see any harm to adding it.

[stifskere]

That just happened to me yesterday, so I upvote this.

[fdncred]

Yup, now is one of the couple times this has happened due to changes at uutils/coreutils.