cosmic-bg: `bin-src` -> `cargo-target-dir`

Avoids hardcoding the path to the binary, making
the packaging more robust.

pkgs/by-name/co/cosmic-bg/package.nix

···
     "prefix"
     (placeholder "out")
     "--set"
-    "bin-src"
-    "target/${stdenv.hostPlatform.rust.cargoShortTarget}/release/cosmic-bg"
+    "cargo-target-dir"
+    "target/${stdenv.hostPlatform.rust.cargoShortTarget}"
   ];
 
   passthru = {