[chore] use CNCF actuated goproxy (open-telemetry#32368)

**Description:**
Use a goproxy host provisioned for this project.

**Related Issue:**
Fixes open-telemetry#32330

.github/workflows/build-and-test.yml

@@ -8,6 +8,7 @@ on:
  pull_request:
 env:
  TEST_RESULTS: testbed/tests/results/junit/results.xml
+ GOPROXY: https://goproxy.cncf.actuated.dev,direct
  # Make sure to exit early if cache segment download times out after 2 minutes.
  # We limit cache download as a whole to 5 minutes.
  SEGMENT_DOWNLOAD_TIMEOUT_MINS: 2