fix(aws-lambda-python): skip default docker build when image passed (#19143)

#18082 added the ability to pass bundling parameters to `PythonFunction`'s. Currently when the `image` parameters is passed both the passed image and the default image are built even though the default image is not used

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

Author: relm923

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

@@ -70,14 +70,13 @@ export class Bundling implements CdkBundlingOptions {
       outputDir: outputPath,
     });
 
-    const defaultImage = DockerImage.fromBuild(path.join(__dirname, '../lib'), {
+    this.image = image ?? DockerImage.fromBuild(path.join(__dirname, '../lib'), {
       buildArgs: {
         ...props.buildArgs ?? {},
         IMAGE: runtime.bundlingImage.image,
       },
       platform: architecture.dockerPlatform,
     });
-    this.image = image ?? defaultImage;
     this.command = ['bash', '-c', chain(bundlingCommands)];
     this.environment = props.environment;
   }

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

@@ -211,6 +211,7 @@ test('Bundling a function with custom bundling image', () => {
     }),
   }));
 
+  expect(DockerImage.fromBuild).toHaveBeenCalledTimes(1);
   expect(DockerImage.fromBuild).toHaveBeenCalledWith(expect.stringMatching(entry));
 });