add Flatcar Container Linux command

[octu0]

I am using a distro called Flatcar Container Linux Flatcar Container Linux

I added support Flatcar.
However, Flatcar's version numbering is a bit unique, so I've made it a separate split from CentOS and Ubuntu.
https://www.flatcar.org/releases#lts-release

$ cat /etc/os-release
NAME="Flatcar Container Linux by Kinvolk"
ID=flatcar
ID_LIKE=coreos
VERSION=3510.3.5
VERSION_ID=3510.3.5
BUILD_ID=2024-07-02-0638
SYSEXT_LEVEL=1.0
PRETTY_NAME="Flatcar Container Linux by Kinvolk 3510.3.5 (LTS 2023)"
ANSI_COLOR="38;5;75"
HOME_URL="https://flatcar.org/"
BUG_REPORT_URL="https://issues.flatcar.org"
FLATCAR_BOARD="amd64-usr"
CPE_NAME="cpe:2.3:o:flatcar-linux:flatcar_linux:3510.3.5:*:*:*:*:*:*:*"

I'll fix it if other fixes are better.