cmd/go: use local go cache for cover_statements script test
Use a test-local directory for GOCACHE in "cover_statements" script
test, as a workaround for issue 64014.
For the portion of this test that verifies that caching works
correctly, the cache should theoretically always behave
reliably/deterministically, however if other tests are concurrently
accessing the cache while this test is running, it can lead to cache
lookup failures, which manifest as a flaky failure. To avoid such
flakes, use a separate isolated GOCACHE for this test.