ci: don't run go get as part of workflow

[wlynch]

This is masking build errors, since go get will auto-modify go.mod.

I noticed this in https://github.com/bradleyfalzon/ghinstallation/pull/169 - the test didn't fail because it re-added jwt/v4 as part of the get, but golangci-lint failed because it didn't do a similar change.