TIL: `rename` doesn't like its source and target to be on different
partitions. This was causing the `hsmtool` tests to fail whenever we ran them
on a different partition than the lightning-dir (e.g., `/dev/shm` for faster
testing), because we made the backup copy in the current working directory.
This changes this and creates the backup next to the original file, which has
a reasonable chance to be on the same partition.
Changelog-Changed: hsmtool: The `hsmtool` now creates its backup copy in the same directory as the original `hsm_secret` file.