update github.com/urfave/cli to v2

github.com/urfave/cli has started using go modules, and has released v2
with some breaking changes to the API. Nothing major, mostly just
cosmetic changes.

I don't think there's any change in behavior that affects us.

I followed the migration guide at [1], then fixed the remaining compiler
errors.

[1] https://github.com/urfave/cli/blob/master/docs/migrate-v1-to-v2.md

bootstrap/integration/main_test.go

@@ -9,7 +9,7 @@ import (
 	"github.com/buildkite/agent/v3/clicommand"
 	"github.com/buildkite/agent/v3/experiments"
 	"github.com/buildkite/bintest"
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 )
 
 func TestMain(m *testing.M) {