buildRustPackage: add isMips64n32 to badPlatforms

Rust is not yet able to target the n32 ABI on mips64.

Let's add `isMips64n32` to the `meta.badPlatforms` of all
derivations created by buildRustPackage.

I use this to automatically detect which packages on my system can
be built for n32 (almost all of them) and build those using n32, and
the few packages (mainly those that depend on boost or rust) that
can't for n64.

pkgs/build-support/rust/build-rust-package/default.nix

···
       "x86_64-redox"
       "wasm32-wasi"
     ];
+    badPlatforms = [
+      # Rust is currently unable to target the n32 ABI
+      lib.systems.inspect.patterns.isMips64n32
+    ];
   } // meta;
 })