[makefile] Fix failing check to verify we're on Arch

Use /etc/os-release instead of /etc/issue in the Makefile, see:
!271 (comment 1232050)

.gitlab-ci.yml

@@ -54,7 +54,6 @@ build_Arch_Linux_package:
     - "chown -R non_root:non_root /home/non_root"
   script:
     # makepkg does not run as root, but it needs sudo to install dependencies
-    - "cat /etc/issue"
     - "cd /home/non_root/build && sudo -u non_root make package_arch"
     - "cd /home/non_root/build && pacman -U ArchLinux/*tar.zst --noconfirm"
     # artifacts must be relative to the working directory (e.g. /builds/leap/bitmask-vpn)

Makefile

@@ -444,7 +444,7 @@ package_deb:
 	@${MAKE} -C build/${PROVIDER} pkg_deb
 
 package_arch:
-	grep -q "Arch Linux" /etc/issue || (echo "Arch Linux system is required to build the Arch Linux package" && exit 1)
+	grep -q "Arch Linux" /etc/os-release || (echo "Arch Linux system is required to build the Arch Linux package" && exit 1)
 	# at least the makepkg binary is mandatory, makepkg is part pacman package
 	(cd ArchLinux && makepkg --syncdeps --noconfirm)