Depends on #7116 and #7119.

In addition to the release testing described in #7116, Spack should do build tests on pull requests made to develop.

• Create a binary package repository with builds for the latest develop.

• Implement PR testing for develop in a similar way to releases -- treat develop as an "ongoing" release, and it should have a default set of test builds defined in a release.yaml (or maybe some other name) a the root of the repository.

• Develop a suitable purge policy for develop. We can't keep around all builds as binary packages, but we can keep around some.

• Implement the ability for authorized users to comment on PRs and trigger builds of specified dependents of the packages in the PR. A user with Spack write access should be able to comment on a PR with syntax like build: <spec> and trigger the build of the specified spec.