Disable performance tests on pull requests

This approach isn't ideal because it won't allow the tests to be
triggered on a PR manually (for example, when new benchmarks are added).
Instead, we might follow the "pr" tag approach that the
mongodb-python-driver Evergreen config uses.

However, I think we should revisit this later if that solution seems
justified. Temporarily reverting this patch in a PR branch may work to
check the performance tests there if need be. Evergreen shows ~$0.30
is wasted when this job runs on a PR since the results aren't used.

Author: timgraham

.evergreen/config.yml

@@ -173,6 +173,7 @@ tasks:
       - func: "teardown csfle"
 
   - name: run-performance-tests
+    patchable: false
     commands:
       - func: "run performance tests"
       - func: "attach performance test results"