libxslt: fix cross compilation (#189766)

* libxslt: fix cross compilation

The derivation is already prepared to disable python
if buildPlatform != targetPlatform.
Additionally, the configure of libxslt needs to be told
`--without-python` to not look for the python interpreter.

Co-authored-by: Christoph Neidahl <[e-mail]>

pkgs/development/libraries/libxslt/default.nix

···
     "--without-debug"
     "--without-mem-debug"
     "--without-debugger"
-  ] ++ lib.optionals pythonSupport [
-    "--with-python=${python}"
+    (lib.withFeatureAs pythonSupport "python" python)
   ] ++ lib.optionals (!cryptoSupport) [
     "--without-crypto"
   ];