Out-of-tree electrs build places the built binary in the wrong directory

[dagurval]

Run it locally on a out-of-tree build and noticed that electrs binary is placed in the wrong place. I mean all other binaries is placed under the <build dir>/src whereas electrs is placed in ./src

I think we should find a way to make build_electrs.py aware of the fact that we are building out-of-tree...

Originally posted by @sickpig in #1869 (comment)