To allow using a custom btcd executable, we allow specifying a path to a
file. If the path is empty, the harness will fall back to compiling one
from scratch.
Previously, rpctest would start a btcd node using the btcd executable
in the environment PATH. This caused difficult-to-find issues where
the code would be tested against an older version of btcd, or another
fork entirely. Now it compiles btcd the first time it is needed and
uses that fresh version when launching nodes.
