osbuilder: Specify version when installing Rust

and update the script in `ci/` accordingly.
When only parts of the Kata Containers repositories are checked out
(e.g. when building with Snap) and no Rust version is provided in
calling `install_rust.sh`, the scripts will attempt to clone the
appropriate repos to read the version, which will fail because the
directories already exist. Since we have read the version already, we
can just specify it.

Signed-off-by: Jakob Naucke <jakob.naucke@ibm.com>

ci/install_rust.sh

@@ -12,5 +12,5 @@ source "${cidir}/lib.sh"
 clone_tests_repo
 
 pushd ${tests_repo_dir}
-.ci/install_rust.sh
+.ci/install_rust.sh ${1:-}
 popd