scripts/installer: fix elementaryOS

c2b907c965 moved UBUNTU_VERSION out
of the ubuntu case and into linuxmint, but linuxmint wasn't the
only Ubuntu-based system which needed it. Restore UBUNTU_VERSION
handling in the ubuntu case.

Break elementaryOS out into its own handling so we can get the
version number handling correct for keyring support.
Tested on an elementaryOS 6.1 VM.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

scripts/installer.sh

@@ -41,11 +41,16 @@ main() {
 		#  - ID: the short name of the OS (e.g. "debian", "freebsd")
 		#  - VERSION_ID: the numeric release version for the OS, if any (e.g. "18.04")
 		#  - VERSION_CODENAME: the codename of the OS release, if any (e.g. "buster")
+		#  - UBUNTU_CODENAME: if it exists, use instead of VERSION_CODENAME
 		. /etc/os-release
 		case "$ID" in
-			ubuntu|pop|neon|zorin|elementary)
+			ubuntu|pop|neon|zorin)
 				OS="ubuntu"
+				if [ "${UBUNTU_CODENAME:-}" != "" ]; then
+				    VERSION="$UBUNTU_CODENAME"
+				else
 				    VERSION="$VERSION_CODENAME"
+				fi
 				PACKAGETYPE="apt"
 				# Third-party keyrings became the preferred method of
 				# installation in Ubuntu 20.04.
@@ -85,6 +90,16 @@ main() {
 					APT_KEY_TYPE="keyring"
 				fi
 				;;
+			elementary)
+				OS="ubuntu"
+				VERSION="$UBUNTU_CODENAME"
+				PACKAGETYPE="apt"
+				if [ "$VERSION_ID" -lt 6 ]; then
+					APT_KEY_TYPE="legacy"
+				else
+					APT_KEY_TYPE="keyring"
+				fi
+				;;
 			parrot)
 				OS="debian"
 				PACKAGETYPE="apt"