fix(lambda-python): asset bundling fails on windows (#19270)

The output directory should always use posix paths since this directory
is being used within a linux docker container.

I did not add any tests because it looks like there is no way to mock the `path` platform detection.

fixes #18861


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packages/@aws-cdk/aws-lambda-python/lib/bundling.ts

@@ -69,7 +69,7 @@ export class Bundling implements CdkBundlingOptions {
       image,
     } = props;
 
-    const outputPath = path.join(AssetStaging.BUNDLING_OUTPUT_DIR, outputPathSuffix);
+    const outputPath = path.posix.join(AssetStaging.BUNDLING_OUTPUT_DIR, outputPathSuffix);
 
     const bundlingCommands = this.createBundlingCommand({
       entry,