rustc: fix >=1.68 host!=build

Our `rustc.nix` adds a `--target` flag for the host when doing a
host!=target build, but neglects to add a `--target` flag for the
buildPlatform when doing a build!=(host==target) build.  This commit
corrects that.

Before rustc 1.68 omitting the --target flag for the buildPlatform
did not cause any problems.  As of rustc 1.68, build!=host without a
--target for the build will fail like below (with hundreds more
"cannot find std::" errors.

```
$ nix build -f . -L pkgsCross.aarch64-multiplatform.rustc
...
Copying stage1 library from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / aarch64-unknown-linux-gnu)
Uplifting stage1 library (x86_64-unknown-linux-gnu -> aarch64-unknown-linux-gnu)
Copying stage2 library from stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / aarch64-unknown-linux-gnu)
Building stage2 tool rust-analyzer-proc-macro-srv (aarch64-unknown-linux-gnu)
   Compiling autocfg v1.1.0
   Compiling libc v0.2.135
   Compiling cfg-if v1.0.0
   Compiling proc-macro2 v1.0.47
   Compiling quote v1.0.21
   Compiling unicode-ident v1.0.5
   Compiling syn v1.0.102
   Compiling once_cell v1.15.0
   Compiling parking_lot_core v0.9.4
   Compiling serde_derive v1.0.145
   Compiling hashbrown v0.12.3
   Compiling scopeguard v1.1.0
   Compiling smallvec v1.10.0
   Compiling log v0.4.17
   Compiling serde v1.0.145
   Compiling rustc-hash v1.1.0
error[E0463]: can't find crate for `std`
error: cannot find macro `println` in this scope
 --> /nix/tmp/nix-build-rustc-aarch64-unknown-linux-gnu-1.68.2.drv-0/rustc-1.68.2-src/vendor/libc-0.2.135/build.rs:7:5
  |
7 |     println!("cargo:rerun-if-changed=build.rs");
  |     ^^^^^^^
error: cannot find macro `println` in this scope
  --> /nix/tmp/nix-build-rustc-aarch64-unknown-linux-gnu-1.68.2.drv-0/rustc-1.68.2-src/vendor/libc-0.2.135/build.rs:16:9
   |
16 |         println!(
   |         ^^^^^^^
error: cannot find macro `println` in this scope
  --> /nix/tmp/nix-build-rustc-aarch64-unknown-linux-gnu-1.68.2.drv-0/rustc-1.68.2-src/vendor/libc-0.2.135/build.rs:29:13
   |
29 |             println!("cargo:rustc-cfg=freebsd10")
   |             ^^^^^^^
```

pkgs/development/compilers/rust/rustc.nix

@@ -79,11 +79,19 @@ in stdenv.mkDerivation rec {
     "--enable-vendor"
     "--build=${rust.toRustTargetSpec stdenv.buildPlatform}"
     "--host=${rust.toRustTargetSpec stdenv.hostPlatform}"
-    # std is built for all platforms in --target. When building a cross-compiler
-    # we need to add the host platform as well so rustc can compile build.rs
-    # scripts.
+    # std is built for all platforms in --target.
     "--target=${concatStringsSep "," ([
       (rust.toRustTargetSpec stdenv.targetPlatform)
+
+    # (build!=target): When cross-building a compiler we need to add
+    # the build platform as well so rustc can compile build.rs
+    # scripts.
+    ] ++ optionals (stdenv.buildPlatform != stdenv.targetPlatform) [
+      (rust.toRustTargetSpec stdenv.buildPlatform)
+
+    # (host!=target): When building a cross-targeting compiler we
+    # need to add the host platform as well so rustc can compile
+    # build.rs scripts.
     ] ++ optionals (stdenv.hostPlatform != stdenv.targetPlatform) [
       (rust.toRustTargetSpec stdenv.hostPlatform)
     ])}"