Repository Level Increasing CIRRUS_TASK_NUMBER

[fkorotkov]

Description

Sometimes it useful to have a monotonically increasing number of a task within the same repository for tagging purposes (of builds and other artifacts). CIRRUS_TASK_ID is not suitable since it is int64 and too big for some tools, plus it doesn't not monotonically increasing.

Context

See #246

Anything Else

Hard to implement with an eventually consistent store when there are a lot of tasks are creating concurrently.

[fkorotkov]

Will be achieved with Starlark as part of #1078