all: update build constraints to Go 1.17

Do it all at once in preparation for Go 1.18 support.

To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.

src/examples/dac/circuitplay_express.go

@@ -1,3 +1,4 @@
+//go:build circuitplay_express
 // +build circuitplay_express
 
 package main

src/examples/dac/pyportal.go

@@ -1,3 +1,4 @@
+//go:build pyportal
 // +build pyportal
 
 package main