kicad: correctly handle share/metainfo and appdata

fixes issue 123299

blindly linking unstable's metainfo resulted in a dangling symlink
this is still share/appdata in the stable package so i linked that

pkgs/applications/science/electronics/kicad/default.nix

@@ -257,9 +257,12 @@ stdenv.mkDerivation rec {
   postInstall = ''
     mkdir -p $out/share
     ln -s ${base}/share/applications $out/share/applications
-    ln -s ${base}/share/metainfo $out/share/metainfo
     ln -s ${base}/share/icons $out/share/icons
     ln -s ${base}/share/mime $out/share/mime
+  '' + optionalString (stable) ''
+    ln -s ${base}/share/appdata $out/share/appdata
+  '' + optionalString (!stable) ''
+    ln -s ${base}/share/metainfo $out/share/metainfo
   '';
 
   # can't run this for each pname