[dagit] Use tabular-nums for in-progress solid execution timing

Summary: I noticed the elapsed time for solid execution in the Gantt status panel doesn't use tabular nums, so the digits jump around a bit. Fix this.

Test Plan: Start a run, verify that the elapsed time display in the status panel has nicer rendering.

Reviewers: bengotow, prha, sashank

Reviewed By: prha

Differential Revision: https://dagster.phacility.com/D5909