Fix build failing on windows in some unexplainable situations

For some reason on windows cpSync throws error with code 'Unknown system error 17' instead of 'ENOENT'. Changing cpSync to async cp fixes the problem

src/build.ts

@@ -52,7 +52,7 @@ export async function build(exit: boolean) {
     }
 
     try {
-        copyAssetData()
+        await copyAssetData()
 
         writeDistManifest(manifestText)
     } catch (error) {

src/config.ts

@@ -4,8 +4,9 @@ import { InputOptions, OutputOptions, Plugin } from "rollup";
 import nodeResolve from "@rollup/plugin-node-resolve";
 import commonjs from "@rollup/plugin-commonjs";
 import typescript from "rollup-plugin-typescript2";
-import { cpSync, readFileSync, writeFileSync } from "node:fs";
+import { readFileSync, writeFileSync } from "node:fs";
 import { cleandir } from "rollup-plugin-cleandir";
+import { cp } from "node:fs/promises";
 
 // needs to be valid and properly cased js identifier
 const preferenceName = z.string()
@@ -237,9 +238,9 @@ export function writeDistManifest(manifestText: string) {
     writeFileSync("dist/gauntlet.toml", manifestText)
 }
 
-export function copyAssetData() {
+export async function copyAssetData() {
     try {
-        cpSync("assets", "dist/assets", { recursive: true });
+        await cp("assets", "dist/assets", { recursive: true });
     } catch (err) {
         if ((err as any).code === 'ENOENT') {
             return;

src/dev.ts

@@ -97,7 +97,7 @@ export async function dev() {
                     break;
                 }
 
-                copyAssetData()
+                await copyAssetData()
 
                 writeDistManifest(manifestText)