[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