[skip changelog] [skip ci] Fix installation script's latest tag determination code (#772)

GitHub changed the HTML of the release page, which resulted in grep finding multiple instances of the pattern, causing the installation script to fail when no version parameter was provided:

$ curl -fsSL https://raw.githubusercontent.com/arduino/arduino-cli/master/install.sh | sh
Installing in /home/per/temporary/bin
ARCH=64bit
OS=Linux
Using curl as download tool
TAG=0.11.0
0.11.0
0.11.0
CLI_DIST=arduino-cli_0.11.0
0.11.0
0.11.0_Linux_64bit.tar.gz
Downloading https://downloads.arduino.cc/arduino-cli/arduino-cli_0.11.0
0.11.0
0.11.0_Linux_64bit.tar.gz
Failed to install arduino-cli

The chosen fix was to expand the grep pattern to something that should only ever occur once on an HTML page.

Author: per1234

install.sh

@@ -84,9 +84,9 @@ checkLatestVersion() {
 	local regex="[0-9][A-Za-z0-9\.-]*"
 	local latest_url="https://github.com/arduino/arduino-cli/releases/latest"
 	if [ "$DOWNLOAD_TOOL" = "curl" ]; then
-		tag=$(curl -SsL $latest_url | grep -o "Release $regex · arduino/arduino-cli" | grep -o "$regex")
+		tag=$(curl -SsL $latest_url | grep -o "<title>Release $regex · arduino/arduino-cli" | grep -o "$regex")
 	elif [ "$DOWNLOAD_TOOL" = "wget" ]; then
-		tag=$(wget -q -O - $latest_url | grep -o "Release $regex · arduino/arduino-cli" | grep -o "$regex")
+		tag=$(wget -q -O - $latest_url | grep -o "<title>Release $regex · arduino/arduino-cli" | grep -o "$regex")
 	fi
 	if [ "x$tag" = "x" ]; then
 		echo "Cannot determine latest tag."