introduce download_reclient gclient custom var

Add download_reclient gclient custom var to control
whether download reclient binary or not.

no behavior change. it's still true by default.

Bug: 448517720
Change-Id: Ifc822f56cdd540c2ade178d9cd498b38e6e81d68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7076620
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Junji Watanabe <jwata@google.com>
Cr-Commit-Position: refs/heads/main@{#1534717}
NOKEYCHECK=True
GitOrigin-RevId: 4f40a15ff36b0156ea2c4931f86ef0c16074f85f