etlas prefers build tool in global path although a custom one is needed

[jneira]

• Bug discovered trying to patch a package that uses alex: typelead/eta-hackage#149
• eta needs a custom version of alex that doesn't add c headers when generate the .hs file from the template .x
• so there is a patched version in eta-hackage suitable for eta
• if you have for any reason alex in the global path etlas picks up it although it drives to compilation errors
• the workaround is set the appropiate alex in etlas global config or as cli param --with-alex path