haskell.compiler: always include python when building with hadrian

We previously thought that we only need python if we were going to run
./boot or using emscripten which implements all its wrappers in
python (and likes to reinvoke them). As it turns out, though, hadrian
likes to invoke python itself for generating certain headers of rts
using a script shipped with the GHC source. This fact was obscured
before, since (presumably) sphinx would propagate python into PATH.

pkgs/development/compilers/ghc/common-hadrian.nix

@@ -397,16 +397,12 @@ stdenv.mkDerivation ({
     perl ghc hadrian bootPkgs.alex bootPkgs.happy bootPkgs.hscolour
     # autoconf and friends are necessary for hadrian to create the bindist
     autoconf automake m4
-  ] ++ lib.optionals (rev != null) [
-    # We need to execute the boot script
+    # Python is used in a few scripts invoked by hadrian to generate e.g. rts headers.
     python3
   ] ++ lib.optionals (stdenv.isDarwin && stdenv.isAarch64) [
     autoSignDarwinBinariesHook
   ] ++ lib.optionals enableDocs [
     sphinx
-  ] ++ lib.optionals targetPlatform.isGhcjs [
-    # emscripten itself is added via depBuildTarget / targetCC
-    python3
   ];
 
   # For building runtime libs