Use .cargo/config.toml instead of .cargo/config

`.cargo/config` will be deprecated in Rust 1.78.

`.cargo/config.toml` has been supported since Rust 1.38.

rust-lang/cargo#13349

Author: proski

Diff for: overlay/mkcrate.nix

@@ -177,7 +177,7 @@ let
 
     configureCargo = ''
       mkdir -p .cargo
-      cat > .cargo/config <<'EOF'
+      cat > .cargo/config.toml <<'EOF'
       [target."${rustBuildTriple}"]
       linker = "${ccForBuild}"
     '' + optionalString (codegenOpts != null && codegenOpts ? "${rustBuildTriple}") (''