The CI job `prep-deps` was broken in #20096 for forks and non-PR builds
(e.g. the `develop` branch builds). Non-PR builds were broken because
of the `set -u` flag, which complained about a PR-specific environment
variable being unset. Fork builds were broken because the draft check
relied upon secrets (which aren't included in CI runs on forks).
The script is now tolerant of missing environment variables, and skips
the draft check for forks.
Certain draft PRs that add new dependencies have been failing because
CI will try to use the GitHub npm registry, which we use for preview
builds. This registry does not have non-preview package versions, so
the installation will fail if new non-preview dependencies are needed.
CI has been updated to only use the GitHub npm registry when preview
builds are detected in the manifest.
