feat: accommodate missing /run/user dir in OCI mode

[dtrudg]

Description of the Pull Request (PR):

On systems where pam-systemd is disabled, or there is other configuration preventing the creation of a /run/user/ directory, we need to use a different location for the OCI runtime state.

Change the runtimeStateDir handling to:

• Use XDG_RUNTIME_DIR if set.
• Otherwise, try to use /run/user/.
• Otherwise, use a location under $TMPDIR.

This fixes or addresses the following GitHub issues:

• Fixes singularity exec --oci fails on node without /run/user/<uid> directory #3393

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[nathanweeks]

Seems to work as advertised in my environment (with no /run/user/$UID directory)

$ mkdir /tmp/run
$ export XDG_RUNTIME_DIR=/tmp/run
$ singularity exec --oci test.oci-sif grep PRETTY_NAME /etc/os-release
INFO:    System configuration does not support cgroup management - starting container in current cgroup
PRETTY_NAME="Ubuntu 24.04.1 LTS"
$ unset XDG_RUNTIME_DIR
$ mkdir /tmp/foo
$ export TMPDIR=/tmp/foo
$ singularity exec --oci test.oci-sif grep PRETTY_NAME /etc/os-release
INFO:    System configuration does not support cgroup management - starting container in current cgroup
INFO:    No /run/user session directory for user. Using "/tmp/foo/singularity-oci-1000" for runtime state.
PRETTY_NAME="Ubuntu 24.04.1 LTS"
$ unset TMPDIR
$ singularity exec --oci test.oci-sif grep PRETTY_NAME /etc/os-release
INFO:    System configuration does not support cgroup management - starting container in current cgroup
INFO:    No /run/user session directory for user. Using "/tmp/singularity-oci-1000" for runtime state.
PRETTY_NAME="Ubuntu 24.04.1 LTS"