chore(deps): Update config to 0.15.8

[Jongy]

Resolves: #2428
Resolves: #2012

config 0.15.8 contains the fix for rust-cli/config-rs#579 which causes the crash mentioned in the ticket here.

I tested the resulting atuin binary manually, in a non-Unicode directory on my Linux box - the atuin binary I use daily (18.3.0) crashes due to PWD being non-Unicode, and the built atuin binary does not. I did not run any further tests (is CI enough for this?) or look if anything in particular changed in config's API across the changed versions.

Checks

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

[akinomyoga]

Does this also fix #2012?

edit: You can find a reduced reproducer of #2012 in comment #2012 (comment).

[Jongy]

@akinomyoga yes, it fixes that issue as well. config will no longer panic if the faulty env var is filtered out; atuin uses Environment::with_prefix("atuin") so environment variables like a, PWD are filtered out. faulty env vars that are filtered in would still cause a crash, but that's just misuse.

[akinomyoga]

Thanks!