[short] skip 'builds pkgsite on clean GOCACHE'

# Test that go doc -http succeeds on a clean GOCACHE (golang/go#80287).
env TEST_GODOC_BUILD_ONLY=1
env GOCACHE=$WORK/clean-gocache

# First run with clean GOCACHE
go doc -http fmt

# Second run with existing GOCACHE
go doc -http fmt