Use deterministic stage target

[HenrikBengtsson]

When running interactively, one might wanna stage and unstage multiple times.

One could make the target folder deterministic so that it can be reused. Maybe something like:

• TMPDIR/conda-stage/PPID/NAME

One could compare the timestamp of the .conda-stage file with the tarball to decide whether one needs to rebuild or not.

For this approach to work, we need and option not to wipe, e.g. --no-cleanup.

[HenrikBengtsson]

An advantage of a process-specific (PID/PPID) stage target is that one could set up TMPDIR cleanup process that aggressively cleans out left-over conda-staged temp folders.