fix: Clean up unexpected tflint-reports, and only upload XML

There's some weird behavior in CI where tflint-reports ends up with a nested tflint-reports directory, leading to a failure to upload (the uploader doesn't handle nested directories, only a list of files).  Plausibly some sort of caching (git fetch vs git clone perhaps), but even then it's not clear how the nesting occurs.  Debugging this would be interesting, but time consuming and inefficient.  Therefore, fix this two ways:
1. Delete any pre-existing tflint-reports directory before creating it
2. Only upload *.xml, not **; if any other stray directories still sneak in, they won't be picked up

Example: https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/amp/-/jobs/3724020834

tflint.yml

@@ -5,6 +5,7 @@ tflint:
     name: registry.gitlab.com/gitlab-com/gl-infra/common-ci-tasks-images/tflint:${GL_ASDF_TFLINT_VERSION}
     entrypoint: [""]
   script:
+    - rm -rf "tflint-reports/"
     - mkdir -p "tflint-reports/"
     # Find all TF files and lint the directories of those files
     - tflint --init -c .tflint.hcl
@@ -32,7 +33,7 @@ tflint:
   artifacts:
     when: always
     reports:
-      junit: tflint-reports/**
+      junit: tflint-reports/*.xml
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH