Avoid `panic!()` when current directory does not exist (#9876)
## Summary
If the shell is currently in a directory that no longer exists, uv will
panic from any command. Panicking is a confusing behavior to those
unfamiliar with Rust and can sometimes make it hard to determine the
true issue.
Closes #9875
## Test Plan
The reproduction steps in the issue report were followed and uv no
longer panics. `uv version` can still successfully print the version if
the directory does exist.
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Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
