Make the OS-specific staticcheck jobs only test stuff that's specialized
for that OS. Do that using a new ./tool/listpkgs program that's a fancy
'go list' with more filtering flags.
Updates tailscale/corp#28679
Change-Id: I790be2e3a0b42b105bd39f68c4b20e217a26de60
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
