misc/get_key.py: load vault file without chdir'ing

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")