Read the task commit from task_info.yaml

[rcannood]

Describe your changes

get_task_info hardcoded the commit, so every task_info.json we have ever
published carries "commit": null:

commit = jsonlite::unbox(NA_character_), # TODO: Add when available in task_info.yaml

It now reads the field:

commit = jsonlite::unbox(task_info_yaml$commit %||% NA_character_),

The %||% is there for the null rather than as a placeholder -- unbox(NULL)
serialises to {}, not null, so dropping it entirely would change the shape of
the output for every task that does not write the field yet:

jsonlite::toJSON(list(commit = jsonlite::unbox(list()$commit)))
#> {"commit":{}}
jsonlite::toJSON(list(commit = jsonlite::unbox(list()$commit %||% NA_character_)))
#> {"commit":null}

A task that does not supply a commit keeps emitting null, which is exactly what it
does today -- no "missing-sha" placeholder, since a gap here should be fixed in
whatever generated the results rather than papered over.

The other half is in the task repos: run_benchmark has to write workflow.commitId
into task_info.yaml. openproblems-bio/task_spatial_simulators#33 does that, and I
will send the same to task_template.

Issue ticket number and link

Closes #942

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