Remove schema checks on Draft

[CodeGat]

Background

The restrictive schema has been in place for Prereleases as the original rationale was that Prereleases were a natural testing ground for eventual Releases.

This has evolved slightly, with developers using Prereleases as sharable testing grounds with no intention of merging them, making the schema unnessecarily restrictive.

There have been other pull requests to build-cd that have weakened the restrictions on the schema (such as #235 and ACCESS-NRI/schema#45) or removed restrictions on Draft PRs (such as #226).

As a logical next step, we are looking to remove all schema restrictions on Draft PRs, giving developers the flexibility to test what they want, if there is no intention to make it a Release.

Implementation Details

• Make check-spack-yaml job conditional on the draft status of the PR.
  • Decide if we still need to do the modulefile dependency checks for Draft PRs.
• Determine how comments will look for draft PRs - there is a possibilty that there is no longer a spack.specs[0]/spack.definitions.ROOT_PACKAGE[0].

Thanks @harshula for the suggestion. Pinging @anton-seaice and @dougiesquire so they know when this is done.