btrfs-progs: user build's python for man pages generation, not host's

Without the change build fails on attributes like
`pkgsCross.aarch64-multiplatform.btrfs-progs` as:

    sphinx-build: cannot execute binary file: Exec format error

pkgs/tools/filesystems/btrfs-progs/default.nix

@@ -1,5 +1,6 @@
 { lib, stdenv, fetchurl
-, pkg-config, python3
+, buildPackages
+, pkg-config
 , zstd
 , acl, attr, e2fsprogs, libuuid, lzo, udev, zlib
 , runCommand, btrfs-progs
@@ -19,7 +20,7 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [
     pkg-config
   ] ++ [
-    (python3.withPackages (ps: with ps; [
+    (buildPackages.python3.withPackages (ps: with ps; [
       sphinx
       sphinx-rtd-theme
     ]))