This PR upgrades our CI images and fixes some affected tests.

- upgrade go-machine-image to premade latest ubuntu LTS (ubuntu-2004:202111-02)

- eliminate go-machine-recent-image (no longer necessary)

- manage GOPATH in GNUMakefile (see https://discuss.circleci.com/t/gopath-is-set-to-multiple-directories/7174)

- fix tcp dial error check (message seems to be OS specific)

- spot check values measured instead of specifically 'RSS' (rss no longer reported in cgroups v2)

- use safe MkdirTemp for generating tmpfiles

NOT applied: (too flakey)

- eliminate setting GOMAXPROCS=1 (build tools were also affected by this setting)

- upgrade resource type for all imanges to large (2C -> 4C)