fix(toolkit-lib): errors from async builders are not caught (#382)

Errors thrown from async functions passed to `fromAssemblyBuilder` would
not be caught correctly:

```ts
await toolkit.fromAssemblyBuilder(async () => {
  // Oops! Not wrapped the right type of error.
  throw new Error(...);
});
```

Due to a missing `await` in the `try/catch` handler.

---
By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license

Author: rix0rrr

packages/@aws-cdk/toolkit-lib/lib/api/cloud-assembly/private/source-builder.ts

@@ -57,9 +57,9 @@ export abstract class CloudAssemblySourceBuilder {
           const env = await execution.defaultEnvVars();
           const assembly = await execution.changeDir(async () =>
             execution.withContext(context.all, env, props.synthOptions ?? {}, async (envWithContext, ctx) =>
-              execution.withEnv(envWithContext, () => {
+              execution.withEnv(envWithContext, async () => {
                 try {
-                  return builder({
+                  return await builder({
                     outdir: execution.outdir,
                     context: ctx,
                   });

packages/@aws-cdk/toolkit-lib/test/api/cloud-assembly/source-builder.test.ts

@@ -37,11 +37,17 @@ describe('fromAssemblyBuilder', () => {
     expect(JSON.stringify(stack)).toContain('amzn-s3-demo-bucket');
   });
 
-  test('errors are wrapped as AssemblyError', async () => {
+  test.each(['sync', 'async'] as const)('errors are wrapped as AssemblyError for %s builder', async (sync) => {
     // GIVEN
-    const cx = await toolkit.fromAssemblyBuilder(() => {
-      throw new Error('a wild error appeared');
-    });
+    const builder = sync === 'sync'
+      ? () => {
+        throw new Error('a wild error appeared');
+      }
+      : async () => {
+        throw new Error('a wild error appeared');
+      };
+
+    const cx = await toolkit.fromAssemblyBuilder(builder);
 
     // WHEN
     try {
@@ -89,7 +95,7 @@ describe('fromAssemblyBuilder', () => {
     });
 
     // WHEN
-    await expect(cx.produce()).rejects.toThrow(/wild error/);
+    await expect(cx.produce()).rejects.toThrow();
 
     // THEN: Don't expect either a read or write lock on the directory afterwards
     expect(await (lock! as any)._currentWriter()).toBeUndefined();