rustc: add missing xz dependency

The "bootstrap" and "installer" crates depend on lzma-sys, which will
build its own version of xz if it can't find the liblzma.pc through
pkg-config.  Even though it's used as a library, xz here is a native
build input, as it is used by the build system rather than the end
product.

pkgs/development/compilers/rust/rustc.nix

@@ -2,7 +2,7 @@
 , llvmShared, llvmSharedForBuild, llvmSharedForHost, llvmSharedForTarget, llvmPackages
 , fetchurl, file, python3
 , darwin, cmake, rust, rustPlatform
-, pkg-config, openssl
+, pkg-config, openssl, xz
 , libiconv
 , which, libffi
 , withBundledLLVM ? false
@@ -41,6 +41,13 @@ in stdenv.mkDerivation rec {
   # See: https://github.com/NixOS/nixpkgs/pull/56540#issuecomment-471624656
   stripDebugList = [ "bin" ];
 
+  # The Rust pkg-config crate does not support prefixed pkg-config executables[1],
+  # but it does support checking these idiosyncratic PKG_CONFIG_${TRIPLE}
+  # environment variables.
+  # [1]: https://github.com/rust-lang/pkg-config-rs/issues/53
+  "PKG_CONFIG_${builtins.replaceStrings ["-"] ["_"] (rust.toRustTarget stdenv.buildPlatform)}" =
+    "${pkgsBuildHost.stdenv.cc.targetPrefix}pkg-config";
+
   NIX_LDFLAGS = toString (
        # when linking stage1 libstd: cc: undefined reference to `__cxa_begin_catch'
        optional (stdenv.isLinux && !withBundledLLVM) "--push-state --as-needed -lstdc++ --pop-state"
@@ -148,9 +155,11 @@ in stdenv.mkDerivation rec {
   # use it for the normal build. This disables cmake in Nix.
   dontUseCmakeConfigure = true;
 
+  depsBuildBuild = [ pkg-config ];
+
   nativeBuildInputs = [
     file python3 rustPlatform.rust.rustc cmake
-    which libffi removeReferencesTo pkg-config
+    which libffi removeReferencesTo pkg-config xz
   ];
 
   buildInputs = [ openssl ]