Avoid removing the build output directory before a new build finishes

Updating drushmake and copy tasks to build into a temp directory and then copy the output on success. This avoids removing the last build's output before the new build is successful.

Note: This changes the behavior of "clean:default" from removing the entire build directory to instead remove only the html output directory (usually, build/html).

@grayside, correct. There could also be a "clean:all" that wipes the entire build directory, but this may be excessive as the default clean behavior. Often a dev instance points to the build html directory, and it's a pain that the site is down during builds and if the build fails.