nixos: Add fontconfig.lib to shell.nix, in LD_LIBRARY_PATH

Otherwise `flutter run -d linux` fails with an error like (wrapped):

```
$ nix-shell --run 'flutter run -d linux'
Launching lib/main.dart on Linux in debug mode...
/nix/store/pnajkjw1fz03z1x27ski1qbx9n3hd4df-binutils-2.39/bin/ld:
  warning: libfontconfig.so.1, needed by
  /home/greg/z/flutter/linux/flutter/ephemeral/libflutter_linux_gtk.so,
  not found (try using -rpath or -rpath-link)
clang-11: error: linker command failed with exit code 1 (use -v to see invocation)
Building Linux application...
Exception: Build process failed
```

Sadly just including it in `nativeBuildInputs` doesn't fix the issue,
so we resort to LD_LIBRARY_PATH.

Author: gnprice

shell.nix

@@ -38,4 +38,8 @@ mkShell {
     android-studio
     android-tools
   ];
+
+  LD_LIBRARY_PATH = lib.makeLibraryPath [
+    fontconfig.lib
+  ];
 }