Conditionally skipping/running tasks

[alexec]

I would be good to be able to conditionally run tasks. This would be a bit like Github actions, e.g.`

- name: foo
   command: sh script
   if: success()

- name: foo
   command: sh script
   if: failure()

- name: foo
   command: sh script
   if: always()

- name: foo
   command: sh script
   if: env.FOO == 1