Fix show_settings tests not to be affected by system configs (#9992)
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## Summary
Override XDG_CONFIG_DIRS in show_settings tests, in order to ensure that
they don't pick system configuration, and therefore fail due to value
mismatches. This specifically addresses test failures on Gentoo where a
default `/etc/xdg/uv/uv.toml` is installed, and users are free to modify
it.
Prior to #9914, we used to set `XDG_CONFIG_DIRS` locally before running
the test suite. However, since the test now wipes the environment, the
problem can no longer be resolved downstream.
## Test Plan
`cargo test` on a Gentoo system (with `/etc/xdg/uv/uv.toml` present).
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Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
