Use backported Rust 1.68 toolchain by default

Use backported Rust 1.68 toolchain by default

Add the kirkstone/rust-1.68 mixin layer from meta-lts-mixins into
the AGL default base so that the newer Rust toolchain is available
by default to all builds.  This seems the most straightforward way
to integrate it and have it available for use in builds that may or
may not include meta-agl-demo.  It also ensures that build shared
state and any binary packagefeed work do not get impacted by having
a mix of different versions across builds.

Bug-AGL: SPEC-4762

Change-Id: I7d01f4a015de6b342b47ef6e78f62da9bb34ec5f
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/28902
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account