haskell.compiler.ghcjs: Don't use upstream config.sub

Before this change the `ghcjs` build fails with:

```
checking host system type... Invalid configuration `js-ghcjs': machine `js-unknown' not recognized
configure: error: …/bin/bash …/configured-ghcjs-src/lib/boot/pkg/unix/config.sub js-ghcjs failed
```

This fails because the `updateAutotoolsGnuConfigScriptsHook` overrides the
`config.sub` in the `unix` boot package to use the newer upstream
version of `config.sub`, but the newer version is incompatible with the
`js-ghcjs` host string used to configure the `unix` package.  The
fix is to undo the override, because the `config.sub` vendored within
`ghc` supports the host string correctly.

pkgs/development/compilers/ghcjs/8.10/default.nix

@@ -78,6 +78,11 @@ in stdenv.mkDerivation {
     ];
     dontConfigure = true;
     dontInstall = true;
+
+    # Newer versions of `config.sub` reject the `js-ghcjs` host string, but the
+    # older `config.sub` filed vendored within `ghc` still works
+    dontUpdateAutotoolsGnuConfigScripts = true;
+
     buildPhase = ''
       export HOME=$TMP
       mkdir $HOME/.cabal