Update build tag constraints

This patch updates all build tag constraints to add the new format,
alongside the old one, to maintain backwards compatibility.

This was done by using `go fmt`.

See https://go.dev/doc/go1.17#gofmt and
https://golang.org/design/draft-gobuild for more details.

dnsoverride.go

@@ -2,6 +2,7 @@
 // This is only used in tests, when the "dnsoverride" tag is active.
 // It requires Go >= 1.8.
 //
+//go:build dnsoverride
 // +build dnsoverride
 
 package main