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).

Drops capability to clean other transient assets inside build/, such as reports. Not sure that I care.