Uninstalls RPM in prep-dev script

[conorsch]

Status

Ready for review

Description of Changes

Follow up to #587.

The dnf install -y <local_file> action will not reinstall a package
if the versions are the same. Since we expect package contents to
change, but not version strings, when running make clone, let's make
sure to uninstall the rpm package in dom0 so that the install action
always takes effect.

Testing

First, let's observe the problem. The steps to reproduce assume that you've already got a version of `securedrop-workstation-config

1. In dev VM, check out main branch in this repo
2. In dom0, run make clone && make prep-dev. This is just to confirm that you've got a recent version of the rpm already present
3. In dom0, confirm file is absent: sudo ls -1 /srv/salt/ | grep sd-test shows nothing
4. In dev VM, run touch dom0/sd-test.sls to create a new file
5. In dom0, run make clone && make prep-dev
6. In dom0, confirm file is still absent: sudo ls -1 /srv/salt/ | grep sd-test shows nothing

Now let's test the fix.

1. In dev VM, check out this feature branch
2. In dev VM, run git status and confirm dom0/sd-test.sls is still present locally (you don't need to commit it)
3. In dom0, confirm file is absent: sudo ls -1 /srv/salt/ | grep sd-test shows nothing
4. In dom0, run make clone && make prep-dev
5. In dom0, confirm file is present: sudo ls -1 /srv/salt/ | grep sd-test shows "sd-test.sls"

Checklist

If you have made code changes

• Linter (make flake8) passes in the development environment (this box may
  be left unchecked, as flake8 also runs in CI)

If you have made changes to the provisioning logic

• All tests (make test) pass in dom0 of a Qubes install

• This PR adds/removes files, and includes required updates to the packaging
  logic in MANIFEST.in and rpm-build/SPECS/securedrop-workstation-dom0-config.spec

[zenmonkeykstop]

Test plan passes, this LGTM!