Update Rust mixin layer directory

Update Rust mixin layer directory

Drop the version number from the Rust mixin layer target directory
in the bblayers.conf template to match the repo manifest change.
Without the version number, we can hopefully avoid any future
changes here if we do end up needing another Rust bump during
Quillback LTS.

Bug-AGL: SPEC-5060

Change-Id: Iddda8a0e76ab4e202b93295dc985b3135c020be1
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/29638
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>