runtime/secret: restore goroutine behavior to proposal

runtime/secret: restore goroutine behavior to proposal

During review of CL 704615, a suggestion was made that spawning a
goroutine inside a call to secret.Do result in a panic. I agreed with
this at the time, but had missed that this had been extensively
discussed on the proposal. Revert the behavior back to what was agreed
upon.

Change-Id: Ifaa9e24bd03ecbd870ae2217137d1a9527c96842
Reviewed-on: https://go-review.googlesource.com/c/go/+/728920
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>