fix(lambda-nodejs): incorrect working directory for local bundling

Execute local bundling from the directory containing the entry file.

Without this change, in a monorepo with multiple `package.json` files
Parcel doesn't look for the right one.

Also fix a regression introduced in aws#9632 for the working directory in
the container.

packages/@aws-cdk/aws-lambda-nodejs/lib/bundlers.ts

@@ -64,6 +64,7 @@ export class LocalBundler implements cdk.ILocalBundling {
         process.stderr, // redirect stdout to stderr
         'inherit', // inherit stderr
       ],
+      cwd: path.dirname(path.join(this.props.projectRoot, this.props.relativeEntryPath)),
     });
     return true;
   }
@@ -107,7 +108,7 @@ export class DockerBundler {
       image,
       command: ['bash', '-c', command],
       environment: props.environment,
-      workingDirectory: path.dirname(path.join(cdk.AssetStaging.BUNDLING_INPUT_DIR, props.relativeEntryPath)),
+      workingDirectory: path.dirname(path.join(cdk.AssetStaging.BUNDLING_INPUT_DIR, props.relativeEntryPath)).replace(/\\/g, '/'), // Always use POSIX paths in the container,
     };
   }
 }

packages/@aws-cdk/aws-lambda-nodejs/test/bundling.test.ts

@@ -241,6 +241,7 @@ test('Local bundling', () => {
     ],
     expect.objectContaining({
       env: expect.objectContaining({ KEY: 'value' }),
+      cwd: '/project/folder',
     }),
   );