Omit duration sec for waiting runner step in < GHES v3.9

[Kesin11]

GET workflow_job API response does not contain created_at field in GHES v3.8, it is added from GHES v3.9.

So it is not possible to calculate the elapsed time the runner is waiting for a job, actions-timeline inserts a dummy step instead.

[github-actions]

Code Metrics Report

	main (7651d9f)	#27 (f451105)	+/-
Coverage	0.0%	0.0%	0.0%
Test Execution Time	-	2s	+2s

Details

  |                     | main (7651d9f) | #27 (f451105) | +/-  |
  |---------------------|----------------|---------------|------|
  | Coverage            |           0.0% |          0.0% | 0.0% |
  |   Files             |              0 |             0 |    0 |
  |   Lines             |              0 |             0 |    0 |
  |   Covered           |              0 |             0 |    0 |
- | Test Execution Time |              - |            2s |  +2s |

dist_size

	main (7651d9f)	#27 (f451105)	+/-
main.js	55byte	55byte	0byte
post.js	1307793byte	1308378byte	585byte

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Reported by octocov