gate telemetry dispatch calls on TELEMETRY_ENABLED env var (#17551)

Because of the switch away from certificates/mTLS, we are having to rework a few things. In the meantime, telemetry jobs are failing. This PR adds a switch to turn all of the telemetry stuff off - to skip it instead. It is meant to be controlled by an org-wide environment variable, which can be applied to individual repos by ops. At the time of submitting this PR, the environment variable is 'false' and no telemetry is being reported.

Authors:
  - Mike Sarahan (https://github.com/msarahan)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #17551

.github/workflows/pr.yaml

@@ -52,6 +52,7 @@ jobs:
       OTEL_SERVICE_NAME: 'pr-cudf'
     steps:
       - name: Telemetry setup
+        if: ${{ vars.TELEMETRY_ENABLED == 'true' }}
         uses: rapidsai/shared-actions/telemetry-dispatch-stash-base-env-vars@main
   changed-files:
     secrets: inherit
@@ -329,7 +330,7 @@ jobs:
   telemetry-summarize:
     runs-on: ubuntu-latest
     needs: pr-builder
-    if: always()
+    if: ${{ vars.TELEMETRY_ENABLED == 'true' && !cancelled() }}
     continue-on-error: true
     steps:
       - name: Load stashed telemetry env vars