Previously path was concatenated using string interpolation. This commit
switches from string interpolation to path interpolation.
The problem is nix will copy the "${directory}" to a new store
object, meaning anything outside its root won't be available.
From the nix manual:
> A path in an interpolated expression is first copied into the Nix
> store, and the resulting string is the store path of the newly created
> store object.
https://nix.dev/manual/nix/2.28/language/string-interpolation#interpolated-expression
ddddd780e0