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.

lntest/bitcoind.go

@@ -1,5 +1,5 @@
-// +build bitcoind
-// +build !notxindex
+//go:build bitcoind && !notxindex
+// +build bitcoind,!notxindex
 
 package lntest