Now that misc/get_key.py checks if the vault file passed to it exists,
we cannot pass paths only resolvable from the root directory. Instead,
use paths that make sense relative to the current directory and avoid
calling chdir when loading the vault file.

Fixes: 77542146 ("Rewrite get_key.py to use click instead of typer")

Typer doesn't work with Click 8[1].

[1] https://github.com/tiangolo/typer/issues/280