provision: Fix virtualenv-clone handling of success-stamp.
provision: Fix virtualenv-clone handling of success-stamp.

Apparently, virtualenv-clone ends up copying the success-stamp file
that we use to track whether a virtualenv was successfully
provisioned, which results in problems if we get a network error in
the pip install stage afterwards.

The comment explains our fix, but basically we just delete
success-stamp after the clone.

Fixes #11301.

rht/zulip,tommyip/zulip,punchagan/zulip,zulip/zulip,tommyip/zulip,synicalsyntax/zulip,shubhamdhama/zulip,zulip/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,kou/zulip,shubhamdhama/zulip,eeshangarg/zulip,eeshangarg/zulip,shubhamdhama/zulip,synicalsyntax/zulip,rishig/zulip,andersk/zulip,andersk/zulip,timabbott/zulip,zulip/zulip,shubhamdhama/zulip,kou/zulip,shubhamdhama/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,andersk/zulip,timabbott/zulip,brainwane/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,zulip/zulip,showell/zulip,rht/zulip,hackerkid/zulip,kou/zulip,shubhamdhama/zulip,showell/zulip,rishig/zulip,showell/zulip,showell/zulip,kou/zulip,showell/zulip,zulip/zulip,rishig/zulip,timabbott/zulip,synicalsyntax/zulip,kou/zulip,hackerkid/zulip,rishig/zulip,rht/zulip,showell/zulip,eeshangarg/zulip,shubhamdhama/zulip,rht/zulip,brainwane/zulip,timabbott/zulip,brainwane/zulip,rishig/zulip,synicalsyntax/zulip,synicalsyntax/zulip,andersk/zulip,timabbott/zulip,rht/zulip,brainwane/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,brainwane/zulip,eeshangarg/zulip,zulip/zulip,timabbott/zulip,tommyip/zulip,tommyip/zulip,punchagan/zulip,tommyip/zulip,punchagan/zulip,andersk/zulip,tommyip/zulip,brainwane/zulip,punchagan/zulip,synicalsyntax/zulip,synicalsyntax/zulip,rishig/zulip,kou/zulip,punchagan/zulip,tommyip/zulip,hackerkid/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,timabbott/zulip,hackerkid/zulip,andersk/zulip,rishig/zulip