all: remove +build tags (#377)

Commit 759be3dad9 bumped our Go
requirement to 1.18 which allows us to drop support for old-style
`+build` tags. This change runs `go fix ./...` to remove them, and
updates some remaining code generators that produced `+build` lines.

internal/gen/buildtest.go

@@ -20,7 +20,7 @@ func NewBuildTest(cfg printer.Config) Interface {
 
 func (b *buildtest) Generate(is []inst.Instruction) ([]byte, error) {
 	b.Printf("// %s\n\n", b.cfg.GeneratedWarning())
-	b.BuildTag("!integration")
+	b.BuildConstraint("!integration")
 	b.NL()
 	b.Printf("package build\n\n")
 	b.Printf("import (\n")