Allow OCI launcher to shell image that is in on-disk OCI bundle format.

[dtrudg]

Building on #1023, allow singularity shell oci-bundle:mybundle to launch the container, with a default shell. The execution of the shell should follow the current action script behavior, i.e. try

• Shell pointed to by SINGULARITY_SHELL env var.
• /bin/bash if in the container rootfs
• /bin/sh

See also: https://github.com/sylabs/singularity/blob/21c48367a9f5956765ad1c16ee88f9a12441beef/internal/pkg/util/fs/files/action_scripts.go#L161