With the new "continue" feature on by default, it turned out that
ctrl-c during a build, or a .do file returning an error, would mark a
target as "built" even though it hadn't been. This would prevent
retrying it when you started minimal/do again. Use a temp file
instead.
It's a little tricky: to prevent accidental recursion, we want to
create a file *before* building, but clean up that file when starting
the next session. And we rename that file to the actual .did file
*after* building successfully.
