.github/workflows: run tests with GOEXPERIMENT=synctest

So those tests don't rot again.
diff --git a/.github/workflows/go_test.yml b/.github/workflows/go_test.yml
index a6fba4c..0a110e8 100644
--- a/.github/workflows/go_test.yml
+++ b/.github/workflows/go_test.yml
@@ -53,5 +53,5 @@
 
       - name: Run tests
         run: |
-          gotestsum --format testname -- ./...
-          gotestsum --format testname -- -race ./...
+          GOEXPERIMENT=synctest gotestsum --format testname -- ./...
+          GOEXPERIMENT=synctest gotestsum --format testname -- -race ./...