chore: add AWSLINT_SAVE to the PR buildspec (#22743)

----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

Author: otaviomacedo

buildspec-pr.yaml

@@ -2,6 +2,16 @@ version: 0.2
 
 # This buildspec is intended to be used by GitHub PR builds.
 
+env:
+  variables:
+    # Update package.json with "exclude" statements for all failing rules.
+    # Whenever a new attribute to any resource in the CFN spec, by default
+    # awslint will block the build because the attribute does not exist in 
+    # the corresponding L2 ('resource-attribute' rule). By ignoring it
+    # automatically, we save on-call time, and users who have lived without
+    # this attribute so far can wait for a bit longer.
+    AWSLINT_SAVE: "true"
+
 phases:
   install:
     commands: