matrix-authentication-service: fix cross compile (#495369)

      --replace-fail ./share/policy.wasm "$out/share/$pname/policy.wasm"

  '';

  preBuild = ''

  preBuild =

    let

      rustTarget = stdenv.hostPlatform.rust.rustcTarget;

      rustTargetUnderscore = builtins.replaceStrings [ "-" ] [ "_" ] rustTarget;

    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

    '';

  # Adapted from https://github.com/element-hq/matrix-authentication-service/blob/v0.20.0/.github/workflows/build.yaml#L75-L84