fix: never overwrite the key

[ellie]

Now that local history is stored encrypted, new_key should not overwrite an existing one. This may be frustrating, but will remove the risk of Atuin generating a new key and the user losing their old one.

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

[ellie]

if we wish to generate a new key, the options are

1. Manually delete it, re run
2. Delete it in code, very explicitly. Rather than overwrite.