Add support for skipping selinux package install

**What do you want?**

I want a flag for k3sup to tell the k3s installer to skip installing the selinux package.

**Why do you want this?**

I am wanting to deploy k3s on top of a ublue-os/ucore bootc image I am custom building and I am pre installing the selinux package in my build process. Pre installing the selinux package is needed since the old rpm-ostree method for fedora coreos of layering the selinux package on top of the existing base image blocks bootc from allowing upgrading.

**Recommended solution**

--skip-selinux this would pass the env var to the install.sh of INSTALL_K3S_SKIP_DOWNLOAD=selinux

Thanks!