Heads-up: once py-cylc-{flow,rose,uiserver} are in the environments, need to add external `sed` on HPCs (especially w/ Intel)

[climbfuji]

Is your feature request related to a problem? Please describe.
PRs #894 / JCSDA/spack#377 are adding three cylc components to the unified-dev and skylab-dev templates. These packages explicitly require sed as a dependency, which doesn't build with Intel (at least on AWS ParallelCluster). Since it's such a basic utility and available on all Linux platforms (and macOS if not caring about gnu-sed`), we must add it to the external packages on the supported tier-1 platforms at least (and consider updating the instructions for new Linux and macOS site configs).

[climbfuji]

Closed via #924