[reclient] Make reclient CIPD package configurable

Allow to specify reclient_package location through
glient's custom variables.
The change will enable configuring test builders
to use experimental version of reclient and run
long-running experiments directly on the builders.

The CL switches 2 test builders to newly introduced
experiment mode.

Bug: b/272003003
Change-Id: If77cfd41b94a23364cd31e78c0fffd5a3d6b9e87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4322482


Commit-Queue: Bozydar Szadkowski <bozydar@google.com>
Reviewed-by: Garrett Beaty <gbeaty@google.com>
Cr-Commit-Position: refs/heads/main@{#1115266}