Patchbot should clean up after sage-cleaner before each ticket

[embray]

In particular if some build failed (or maybe even if it succeeded), sage-cleaner can leave enough crap in its spawned_processes file under $DOT_SAGE/temp that almost any process that ends up getting spawned is immediately killed, along with the patchbot itself, by sage-cleaner, just because there are so many PIDs in there it's almost inevitable one will be reused.

In fact we should probably just clean all of $DOT_SAGE/temp before each run.

[slel]

Possibly related:

• sage-devel, 2020-07: patchbot fails with disk full

[mwageringel]

In my case, I ran out of space in /tmp, not in $DOT_SAGE/temp (which would have been on a different partition). In fact, $DOT_SAGE/temp is empty, on my patchbot client.