Auto merge of #22997 - pnkfelix:make-timestamps-robust-to-concurrent-edits, r=nikomat...

Auto merge of #22997 - pnkfelix:make-timestamps-robust-to-concurrent-edits, r=nikomatsakis

Make build timestamp files robust in face of concurrent source modification.

Strategy: If the end goal is to touch e.g. `stamp.std`, then we first touch `stamp.std.start_time` before doing anything else.  Then when the receipe finishes, we touch `stamp.std` using the timestamp from `stamp.std.start_time` as the reference time, and remove `stamp.std.start_time`.

Fix #6518.