make: use low-scrypt to speed up unit-race

This predominately affects the lnwallet unit tests which are the bulk of
the execution time.

lnwallet/btcwallet/btcwallet_rpctest.go

@@ -1,4 +1,4 @@
-// +build rpctest
+// +build rpctest lowscrypt
 
 package btcwallet