scripts/installer: support LinuxMint Debian.

The primary distribution for LinuxMint is based on Ubuntu,
but there is an alternate Debian-based distribution called
LMDE. Both variations identify themselves as "linuxmint"

We added UBUNTU_VERSION to the Ubuntu handling for linuxmint,
the only distribution so far found to do this. Instead, split
linuxmint out into its own case and use either UBUNTU_VERSION
or DEBIAN_VERSION, whichever is present.

Tested on an LMDE 5 (elsie) VM.

Updates https://github.com/tailscale/tailscale/issues/2915

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

scripts/installer.sh

@@ -41,16 +41,11 @@ 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, as in linuxmint, use instead of VERSION_CODENAME
 		. /etc/os-release
 		case "$ID" in
-			ubuntu|pop|neon|zorin|elementary|linuxmint)
+			ubuntu|pop|neon|zorin|elementary)
 				OS="ubuntu"
-				if [ "${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.
@@ -72,6 +67,24 @@ main() {
 					APT_KEY_TYPE="keyring"
 				fi
 				;;
+			linuxmint)
+				if [ "${UBUNTU_CODENAME:-}" != "" ]; then
+				    OS="ubuntu"
+				    VERSION="$UBUNTU_CODENAME"
+				elif [ "${DEBIAN_CODENAME:-}" != "" ]; then
+				    OS="debian"
+				    VERSION="$DEBIAN_CODENAME"
+				else
+				    OS="ubuntu"
+				    VERSION="$VERSION_CODENAME"
+				fi
+				PACKAGETYPE="apt"
+				if [ "$VERSION_ID" -lt 5 ]; then
+					APT_KEY_TYPE="legacy"
+				else
+					APT_KEY_TYPE="keyring"
+				fi
+				;;
 			parrot)
 				OS="debian"
 				PACKAGETYPE="apt"