cargo-tarpaulin: disable aarch64 and i686 build

Those CPU architectures seem to be unsupported as several constructs[1][2]
are only supported for `x86_64`.

[1] d7e0deef8c/src/breakpoint.rs (L7-L8)
[2] d7e0deef8c/src/process_handling/linux.rs (L37)

pkgs/development/tools/analysis/cargo-tarpaulin/default.nix

@@ -25,6 +25,6 @@ rustPlatform.buildRustPackage rec {
     homepage = "https://github.com/xd009642/tarpaulin";
     license = with licenses; [ mit /* or */ asl20 ];
     maintainers = with maintainers; [ hugoreeves ];
-    platforms = platforms.linux;
+    platforms = [ "x86_64-linux" ];
   };
 }