ci: fix ci stats upload condition

The condition I suggested in rust-lang#61632 was not correct and it errors out
while evaluating. This fixes the condition.

Example of a failure:
https://dev.azure.com/rust-lang/rust/_build/results?buildId=543

.azure-pipelines/steps/run.yml

@@ -155,6 +155,6 @@ steps:
 - bash: aws s3 cp --acl public-read cpu-usage.csv s3://$DEPLOY_BUCKET/rustc-builds/$BUILD_SOURCEVERSION/cpu-$SYSTEM_JOBNAME.csv
   env:
     AWS_SECRET_ACCESS_KEY: $(AWS_SECRET_ACCESS_KEY)
-  condition: contains(variables, 'AWS_SECRET_ACCESS_KEY')
+  condition: variables['AWS_SECRET_ACCESS_KEY']
   continueOnError: true
   displayName: Upload CPU usage statistics