Github's matrix runner formats the race variant as '(amd64, true)' if we
use race=true. So, change the way the variable is defined so that it says
'(amd64, race)' even if that makes the if statements a bit more complex.
Signed-off-by: David Anderson <danderson@tailscale.com>
Instead of having a dozen files that contribute CI steps with
inconsistent configs, this one file lists out everything that,
for us, constitutes "a CI run". It also enables the slack
notification webhook to notify us exactly once on a mass breakage,
rather than once for every sub-job that fails.
Signed-off-by: David Anderson <danderson@tailscale.com>
