Run "stringer" using "go run"

In the modern Go Modules-based toolchain we can avoid the need to globally
install this tool first by running it this way. As a bonus, the toolchain
will also install the version of the module we have specified in go.mod,
thereby locking us in to a particular version until we intentionally
upgrade.

The other third-party generator tools we use here aren't written in Go and
so we can't do the same for those right now, but maybe we'll find a nicer
way to handle those later too.

hclsyntax/generate.go

@@ -9,4 +9,4 @@ package hclsyntax
 //go:generate gofmt -w scan_tokens.go
 //go:generate ragel -Z scan_string_lit.rl
 //go:generate gofmt -w scan_string_lit.go
-//go:generate stringer -type TokenType -output token_type_string.go
+//go:generate go run golang.org/x/tools/cmd/stringer -type TokenType -output token_type_string.go

json/scanner.go

@@ -10,7 +10,7 @@ import (
  "github.com/hashicorp/hcl/v2"
 )
 
-//go:generate stringer -type tokenType scanner.go
+//go:generate go run golang.org/x/tools/cmd/stringer -type tokenType scanner.go
 type tokenType rune
 
 const (