matrix-authentication-service: fix cross compilation (#518426)

pkgs/by-name/ma/matrix-authentication-service/package.nix

@@ -14,6 +14,7 @@
   cctools,
   nix-update-script,
   versionCheckHook,
+  buildPackages,
 }:
 
 rustPlatform.buildRustPackage (finalAttrs: {
@@ -52,6 +53,8 @@ rustPlatform.buildRustPackage (finalAttrs: {
     zstd
   ];
 
+  depsBuildBuild = [ buildPackages.stdenv.cc ];
+
   env = {
     ZSTD_SYS_USE_PKG_CONFIG = true;
     VERGEN_GIT_DESCRIBE = finalAttrs.version;
@@ -74,23 +77,16 @@ rustPlatform.buildRustPackage (finalAttrs: {
 
   preBuild =
     let
-      rustTarget = stdenv.hostPlatform.rust.rustcTarget;
-      rustTargetUnderscore = builtins.replaceStrings [ "-" ] [ "_" ] rustTarget;
+      buildTarget = stdenv.buildPlatform.rust.rustcTarget;
+      buildTargetUnderscore = lib.replaceString "-" "_" buildTarget;
     in
     ''
       make -C policies
       (cd "$npmRoot" && npm run build)
 
-      # Fix aws-lc-sys cross-compilation:
-      # The cc crate looks for "aarch64-linux-gnu-gcc")
-      # when CC is unset and TARGET != HOST, but Nix's cross-compiler is
-      # named "aarch64-unknown-linux-gnu-gcc" (with vendor).
-      # We set the target-specific CC_<target> variable so the cc crate
-      # and aws-lc-sys find the correct cross-compiler, then unset the
-      # generic CC so aws-lc-sys doesn't misassign it.
-      export CC_${rustTargetUnderscore}=$CC
-      export CXX_${rustTargetUnderscore}=$CXX
-      unset CC CXX
+      # Fix aws-lc-sys cross-compilation
+      export CC_${buildTargetUnderscore}=$CC_FOR_BUILD
+      export CXX_${buildTargetUnderscore}=$CXX_FOR_BUILD
     '';
 
   # Adapted from https://github.com/element-hq/matrix-authentication-service/blob/v0.20.0/.github/workflows/build.yaml#L75-L84