chore(core): fix bug introduced in recent PR (#12880)

In #12765, a bug was introduced where there was a mismatch between
the file entry written to the asset manifest for the stack template,
and the template location written to the cloud assembly, thereby
causing all deployments to fail.

The error was "Access Denied" (because that's what S3 says when it
doesn't want to tell you "File not found").

Fix the inconsistency.


----

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

packages/@aws-cdk/core/lib/stack-synthesizers/default-synthesizer.ts

@@ -449,7 +449,12 @@ export class DefaultStackSynthesizer extends StackSynthesizer {
     //
     // Instead, we'll have a protocol with the CLI that we put an 's3://.../...' URL here, and the CLI
     // is going to resolve it to the correct 'https://.../' URL before it gives it to CloudFormation.
-    return `s3://${this.bucketName}/${this.bucketPrefix}${sourceHash}`;
+    //
+    // ALSO: it would be great to reuse the return value of `addFileAsset()` here, except those contain
+    // CloudFormation REFERENCES to locations, not actual locations (can contain `{ Ref: AWS::Region }` and
+    // `{ Ref: SomeParameter }` etc). We therefore have to duplicate some logic here :(.
+    const extension = path.extname(this.stack.templateFile);
+    return `s3://${this.bucketName}/${this.bucketPrefix}${sourceHash}${extension}`;
   }
 
   /**

packages/@aws-cdk/core/test/stack-synthesis/new-style-synthesis.test.ts

@@ -36,9 +36,9 @@ nodeunitShim({
     // THEN -- the S3 url is advertised on the stack artifact
     const stackArtifact = asm.getStackArtifact('Stack');
 
-    const templateHash = last(stackArtifact.stackTemplateAssetObjectUrl?.split('/'));
+    const templateObjectKey = last(stackArtifact.stackTemplateAssetObjectUrl?.split('/'));
 
-    test.equals(stackArtifact.stackTemplateAssetObjectUrl, `s3://cdk-hnb659fds-assets-\${AWS::AccountId}-\${AWS::Region}/${templateHash}`);
+    test.equals(stackArtifact.stackTemplateAssetObjectUrl, `s3://cdk-hnb659fds-assets-\${AWS::AccountId}-\${AWS::Region}/${templateObjectKey}`);
 
     // THEN - the template is in the asset manifest
     const manifestArtifact = asm.artifacts.filter(isAssetManifest)[0];
@@ -52,7 +52,7 @@ nodeunitShim({
       destinations: {
         'current_account-current_region': {
           bucketName: 'cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}',
-          objectKey: templateHash + '.json',
+          objectKey: templateObjectKey,
           assumeRoleArn: 'arn:${AWS::Partition}:iam::${AWS::AccountId}:role/cdk-hnb659fds-file-publishing-role-${AWS::AccountId}-${AWS::Region}',
         },
       },