Currently cipd packages listed in src/DEPS are being created and
installed in symlink mode (see bug for more info). This CL adds `$OverrideInstallMode copy` to the ensure files for cipd packages installed during gclient runhooks. `$OverrideInstallMode` was
implemented here, https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3124202,
so that every cipd package can be installed directly to the installation
target, instead of being symlinked to an executable inside the cache/builder/.cipd dir.

The added `$OverrideInstallMode copy` can be removed once all cipd
packages themselves or the default mode is updated to be created in
copy mode.

Bug: 1329641
Change-Id: I68dc718e20f193c384d005c9b6b87603753943d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3673698Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Stephanie Kim <kimstephanie@google.com>