* update go version to 1.15.11 in github workflows
Signed-off-by: hexilee <i@hexilee.me>

* downgrade go version of test/cmd/e2e_helper to 1.15
Signed-off-by: hexilee <i@hexilee.me>
Co-authored-by: Ti Chi Robot <71242396+ti-chi-bot@users.noreply.github.com>