Currently, the bootstrap code from configure gets bootstrapped task info
from the taskgraph, and then invokes `mach artifact toolchain` which...
does the same again. That cumulatively wastes a noticeable amount of
time, especially on Windows.

So to avoid repeating this work that the configure side already did,
we do a little more on the configure side (resolution from index to
task-id), and just give a (task-id, artifact path) pair to `mach
artifact toolchain`. The added code to `mach artifact toolchain` is
actually ironically very similar to the code that was removed in
bug 1687594.

And now that mach bootstrap uses the configure code, it benefits from
this change as well.

Differential Revision: https://phabricator.services.mozilla.com/D135311