fix source root locations.

These have changed following commit
c3255fe8ec.

See also:

- https://github.com/NixOS/nixpkgs/pull/31004
- https://github.com/NixOS/nixpkgs/pull/31106

pkgs/development/libraries/bobcat/default.nix

@@ -16,7 +16,9 @@ stdenv.mkDerivation rec {
   buildInputs = [ libmilter libX11 openssl readline utillinux ];
   nativeBuildInputs = [ icmake yodl ];
 
-  sourceRoot = "bobcat-${version}-src/bobcat";
+  setSourceRoot = ''
+    sourceRoot=$(echo */bobcat)
+  '';
 
   postPatch = ''
     substituteInPlace INSTALL.im --replace /usr $out