Rust build-support: fixing a compilation error in some crates (such as proc-macro2)

pkgs/build-support/rust/build-rust-crate/configure-crate.nix

@@ -71,6 +71,8 @@ in ''
   export CARGO_PKG_VERSION_MAJOR=${builtins.elemAt version 0}
   export CARGO_PKG_VERSION_MINOR=${builtins.elemAt version 1}
   export CARGO_PKG_VERSION_PATCH=${builtins.elemAt version 2}
+  export NUM_JOBS=1
+  export RUSTC="rustc"
   if [[ -n "${versionPre}" ]]; then
     export CARGO_PKG_VERSION_PRE="${versionPre}"
   fi

pkgs/build-support/rust/build-rust-crate/install-crate.nix

@@ -20,9 +20,11 @@ crateName: metadata:
     mkdir -p $out/lib
     cp -r target/build/* $out/lib # */
   fi
-  if [[ "$(ls -A target/bin)" ]]; then
-    mkdir -p $out/bin
-    cp -P target/bin/* $out/bin # */
+  if [[ -d target/bin ]]; then
+    if [[ "$(ls -A target/bin)" ]]; then
+      mkdir -p $out/bin
+      cp -P target/bin/* $out/bin # */
+    fi
   fi
   runHook postInstall
 ''