Local manifests are mostly useful for various kinds of pipelines, e.g. for
merge requests and branch testing. They can use rules:if syntax to modify a
variable that is later used in shell. Due to the way variable interpolation
works in GitLab runners, the value itself cannot use further shell constructs,
like sub-shell substitution.

Introduce CI_ONIRO_REPO_WORKSPACE_LOCAL_MANIFEST_INTERPRETER variable, which
defaults to "echo" and allows using a custom interpreter. One possible use-case
is to use "eval" as interpreter and an arbitrary bash program, which is meant
to print the new local manifest, as value.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>