chore(deps): update whoami dependency to v2 (#3118)

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In the [2.0.0](https://github.com/ardaku/whoami/releases/tag/v2.0.0)
series, `whoami` removed all infallible function variants, and removed
the `fallible` module, moving those functions to the root module.

Therefore, I replaced `whoami::fallible::hostname` with
`whoami::hostname` (the same function with the same signature, just
moved to the root module).

For `whoami::username`, the infallible function that `atuin` was using
before is gone, and we must add error handling. I chose to fall back to
the string `"unknown-user"` if getting the username fails, just as
`"unknown-host"` is already the fallback when getting the hostname
fails. This seemed reasonable to me, but it’s worth double-checking if
there could be any unintended consequences, especially if `unknown-user`
happens to be a real, valid username on the system. The alternatives I
can see would be to panic on failure or to amend the signature of
`get_username()` and all of its call sites with some kind of more
graceful error handling (what?).

## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [x] I have checked that there are no existing pull requests for the
same thing

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