multi: add golang 1.17 compatible build tags

With go 1.17 a change to the build flags was implemented:
https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md

The formatter now automatically adds the forward-compatible build tag
format and the linter checks for them, so we need to include them in our
code.

sweep/defaults.go

@@ -1,3 +1,4 @@
+//go:build !rpctest
 // +build !rpctest
 
 package sweep