* Revert "Saving and restoring the go cache was an experiment to reduce build times, but it worked too inconsistently. (#10493)"

This reverts commit 1e72bb1c.

* make ci-config

* ci: update default go image to use docker mirror

* bump to go1.15.11

- Previously there was version skew: 1.15.10 and 1.15.4 were in use.