Always compile xtask with explicit target (#3244)
Always compile xtask with explicit target (#3244)

This prevents the cargo cache from being overwritten when some other
build command is run. Currently, if you run xtask and it runs some other
command, sometimes it invalidates its own binary in the cargo cache,
which means the next time it is invoked it will have to be rebuilt
again, which takes a few tens of extra seconds each time. Now it is
instant instead.
project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak