Nightly build script changes to integrate with GHA

[bdunne]

Remove RPM build and Container build from nightly-build.sh

• RPM and container builds are handled by github actions now.

Trigger nightly builds from Github Actions

• This will just start the appliance build. Building all platforms would exceed the job timeout. That's for a future PR.
• Use my fork of imagefactory since the official one seems unmaintained and I have a few changes to work on python 3.9 https://github.com/redhat-imaging/imagefactory/pull/454
• Use self-hosted to run on our build VM since we can't build appliances in GHA

[miq-bot]

Checked commits https://github.com/bdunne/manageiq-appliance-build/compare/901a51bbde32608c82a2c3c66aee247ead37ccdc~...b3a630438c21adea2173446b2109b160c94bfba9 with ruby 2.6.10, rubocop 1.28.2, haml-lint 0.35.0, and yamllint 0 files checked, 0 offenses detected Everything looks fine. :cake: