In later versions of macOS (e.g. Ventura), the command used to get a
runtime directory (e.g. `getconf DARWIN_USER_TEMP_DIR`) returns a
trailing slash.
When using a configuration like:
```
sops.defaultSecretsMountPoint = "%r/secrets.d";
```
The final path is going to contain a double slash in the suffix of the
path, an example:
```
/var/<random>/<hash>//secrets.d
```
This commit ensures that the runtime dir will get the trailing '/'
character removed.
On the latest version of macOS (Ventura 13.4 as of this date), this
change ownership will always fail with the error:
> Failed to mount filesystem for secrets: Cannot change owner/group of '.../secrets.d' to 0/0: chown .../secrets.d: operation not permitted
