jobs: get rid of BPO_WIP_REPO_{URL,ARG} vars

Previous code set these shell variables for the sourcehut job service:

  BPO_WIP_REPO_URL: """ + shlex.quote(url_repo_wip) + """
  BPO_WIP_REPO_ARG: '-mp """ + shlex.quote(url_repo_wip) + """'

...and local job service:

  BPO_WIP_REPO_URL="" # empty, because we copy it instead
  BPO_WIP_REPO_ARG="" # empty, because we copy it instead

And then used them later on in the tasks describing the jobs. When they
were used, it was hard to understand what the actual value would be, and
the behavior differed greatly from local job service (testsuite) and
sourcehut, so it lead to bugs like issue 141.

Remove these variables, and call bpo.helpers.pmb.get_pmos_mirror()
instead.

Let the local job service always use the real server's WIP repo URL to
ensure these code paths get tested with the local job service too. To
make this work:
* When building packages: having the real WIP repo mirror enabled in
  addition to having the local WIP packages copied to pmbootstrap's work
  dir should not cause any problems as long as it exists (so not when
  testing staging branches).
* When signing the APKINDEX: download the unsigned APKINDEX from the
  WIP repo mirror first to test that code path. Then remove it and copy
  the local unsigned APKINDEX in its place.