Improve rc file detection based on rustup (#5026)
## Summary
Brings in some learnings from `rustup`:
https://github.com/rust-lang/rustup/blob/fede22fea7b160868cece632bd213e6d72f8912f/src/cli/self_update/shell.rs#L197.
For example: we only need to write to `.zshenv` (but we have to respect
`ZDOTDIR`). Additionally, for Fish, we need to respect `XDG_CONFIG_HOME`
