enable important_pkg on OSX

[timotheecour]

• same as azure-pipelines: test packages on more configuration #13348 but only for OSX
• as Test packages on Linux #13921 noticed, windows was slow but CI here shows this isnt' case for OSX so won't be a bottleneck (except for taking up 1 CI spot):

nim-lang.Nim (packages Linux_amd64_pkg) Successful in 26m — packages Linux_amd64_pkg succeeded
nim-lang.Nim (packages OSX_amd64_pkg) Successful in 20m — packages OSX_amd64_pkg succeeded

future PR

re-enable also windows in a way that doesn't slow down CI on average eg; multiple ways, eg:

• using a probabilistic test (if rand() > 0.1: skip)
• using pipeline schedules (see https://docs.microsoft.com/en-us/azure/devops/pipelines/process/scheduled-triggers?view=azure-devops&tabs=yaml)

- cron: "0 0 * * *"
  displayName: Daily midnight build

• always run for push, but not for pull_request
• run if commit msg contains [nim_ci_all] (using exact same logic as done in [WIP] fix #6999 (allow [nimDocOnly] in commit msg); fix #13477 (flaky nodejs install); log PR url (+other info) in runCI logs #13556)