scripts/installer.sh: use .asc suffix for armored debian gpg key URL.

Fixes #2512

Signed-off-by: David Anderson <danderson@tailscale.com>

scripts/installer.sh

@@ -373,7 +373,7 @@ main() {
 			$SUDO mkdir -p --mode=0755 /usr/share/keyrings
 			case "$APT_KEY_TYPE" in
 				legacy)
-					$CURL "https://pkgs.tailscale.com/stable/$OS/$VERSION.gpg" | $SUDO apt-key add -
+					$CURL "https://pkgs.tailscale.com/stable/$OS/$VERSION.asc" | $SUDO apt-key add -
 					$CURL "https://pkgs.tailscale.com/stable/$OS/$VERSION.list" | $SUDO tee /etc/apt/sources.list.d/tailscale.list
 				;;
 				keyring)