treewide: Simplify some doCheck calls

In anticipation of what I outline in #33599, I only simplify exactly those
`doCheck`s which are equal to `hostPlatform != buildPlatform`. I also stick a
comment next to them so I can grep for them later.

pkgs/development/libraries/libpng/default.nix

@@ -30,7 +30,7 @@ in stdenv.mkDerivation rec {
 
   # it's hard to cross-run tests and some check programs didn't compile anyway
   makeFlags = stdenv.lib.optional (!doCheck) "check_PROGRAMS=";
-  doCheck = hostPlatform == buildPlatform;
+  doCheck = true; # not cross;
 
   passthru = { inherit zlib; };