get-facts: Make os-release parser more generic

This adds support for quoted values (which are allowed by the file
format) and also for `ID_LIKE` for systems that fork NixOS.

Author: dasJ

src/get-facts.sh

@@ -3,13 +3,13 @@ set -efu "${enable_debug:-}"
 has() {
   command -v "$1" >/dev/null && echo "y" || echo "n"
 }
-is_nixos=$(if test -f /etc/os-release && grep -q ID=nixos /etc/os-release; then echo "y"; else echo "n"; fi)
+is_nixos=$(if test -f /etc/os-release && grep -Eq 'ID(_LIKE)?="?nixos"?' /etc/os-release; then echo "y"; else echo "n"; fi)
 cat <<FACTS
 is_os=$(uname)
 is_arch=$(uname -m)
 is_kexec=$(if test -f /etc/is_kexec; then echo "y"; else echo "n"; fi)
 is_nixos=$is_nixos
-is_installer=$(if [ "$is_nixos" = "y" ] && grep -q VARIANT_ID=installer /etc/os-release; then echo "y"; else echo "n"; fi)
+is_installer=$(if [ "$is_nixos" = "y" ] && grep -Eq 'VARIANT_ID="?installer"?' /etc/os-release; then echo "y"; else echo "n"; fi)
 is_container=$(if [ "$(has systemd-detect-virt)" = "y" ]; then systemd-detect-virt --container; else echo "none"; fi)
 has_ipv6_only=$(if [ "$(has ip)" = "n" ] || ip r g 1 >/dev/null 2>/dev/null || ! ip -6 r g :: >/dev/null 2>/dev/null; then echo "n"; else echo "y"; fi)
 has_tar=$(has tar)