To avoid the need for two different versions of plan9port in the CI
pipeline, just move the plumbing sanity check into installCheckPhase.
For local iteration you'd have a plumber running anyways, so the short
build time of the plumb derivation isn't really a necessity.
Change-Id: Ib0ba5fa2d8107530fb7d1144cf7395f4f5a3cd12
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13035
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: sterni <sternenseemann@systemli.org>
623ebc615f