fix: preserve /var/tmp for akmods builds

[bsherman]

This PR fixes a problem I noticed when I switched my "custom kmod image" to be based on this nvidia image rather than the stock silverblue image.

Problem

akmods fail to build with message Unable to open temp file: Permission denied" if the FROM image is one of these ublue-os nvidia images.

Explanation

The original silverblue image includes /var/tmp with normal sticky bit perms, while this image removes /var as part of build cleanup.

The lack of /var/tmp causes rpmbuilds (eg, akmod builds) to fail on any container builds which use ublue-os/nvidia images as their base because the akmod build will create /var/tmp using the default umask (0022). /var/tmp then lacks proper permissions, and akmods fail to create a tmp dir for builds, since /var/tmp gets created by root, but akmods builds run as a user.

Solution

Preserve /var/tmp with correct permissions.

The interesting thing is, I had assumed that the rm -rf /var/* was the cause of the problem, but, ostree container commit actually removes /var from the image even when it still contains files.

See the code blocks for contrast of /var/tmp filesystem state on the respective images and ostree container commit behavior.

stock silverblue /var/tmp state

$ podman run --rm -ti  quay.io/fedora-ostree-desktops/silverblue:37 bash
bash-5.2# umask
0022
bash-5.2# ls -al /var
total 0
drwxr-xr-x. 1 root root  6 Jan  1  1970 .
dr-xr-xr-x. 1 root root 18 Feb 20 16:18 ..
drwxrwxrwt. 1 root root  0 Jan  1  1970 tmp

ublue-os/nvidia silverblue /var/tmp state

$ podman run --rm -ti ghcr.io/ublue-os/silverblue-nvidia:37 bash
bash-5.2# umask
0022
bash-5.2# ls -al /var
ls: cannot access '/var': No such file or directory

stock silverblue ostree container commit behavior

$ podman run --rm -ti  quay.io/fedora-ostree-desktops/silverblue:37 bash
bash-5.2# touch /var/tmp/preserve-me-ostree
bash-5.2# ls -al /var/tmp/
total 0
drwxrwxrwt. 1 root root 36 Feb 20 17:56 .
drwxr-xr-x. 1 root root  6 Jan  1  1970 ..
-rw-r--r--. 1 root root  0 Feb 20 17:56 preserve-me-ostree
bash-5.2# ostree container commit
bash-5.2# ls -al /var/tmp/
ls: cannot access '/var/tmp/': No such file or directory

[castrojo]

Confirmed in chat with bsherman that this was tested locally!