* ci: optimize build workflows
- use go version from go.mod instead of hardcoding
- group platforms by OS for parallel builds
- remove legacy try-build targets
* fix: checkout before setup-go in all workflows
setup-go needs go.mod to be present, so checkout must happen first
* chore: remove deprecated // +build syntax
go 1.17+ uses //go:build, the old syntax is no longer needed
* simplify: remove nofuse tag from CI workflows
- workflows now rely on platform build constraints
- keep make nofuse target for manual builds
- remove unused appveyor.yml
* ci: remove legacy travis variable and fix gateway-conformance
- remove TRAVIS env variable from 4 workflows
- fix gateway-conformance checkout path to match working-directory
- replace deprecated cache-go-action with built-in setup-go caching