path values only get copied to the store when used with string interpolation (e.g. "${./some/path}"). If you instead stringify them with toString, then they are not copied.

E.g.

"${./foo}"
=> "/nix/store/r1l6z113nnzx44iiqp728f50lpczp577-foo"

toString ./foo
=> "/home/matt/foo"

./foo
=> /home/matt/foo