[skip changelog] Improve install script's check for conflicting installation in $PATH (#822)

* Make install script's check for conflicting $PATH installation more reliable

Previously, the comparison between the install script's target path and the path of an existing installation in $PATH could easily give a false positive, producing a spurious "An existing arduino-cli was found at..." error. The solution is to fully resolve the paths before comparing them.

* Don't fail installation script when other installation in $PATH is detected

Previously, when a different installation of Arduino CLI was detected in $PATH by the installation script, the error message "Failed to install arduino-cli" would be printed and the script would have exit status 1.

It is wise that the script warns of this situation ("An existing arduino-cli was found at..."), but the "Failed to install arduino-cli" error message was misleading, since Arduino CLI was indeed installed. It is also incorrect to have an error exit status in this situation, since the user may intend to have multiple installations and provide the full path when using the new installation.

install.sh

@@ -191,8 +191,13 @@ testVersion() {
 	CLI="$(which $PROJECT_NAME)"
 	if [ "$?" = "1" ]; then
 		echo "$PROJECT_NAME not found. You might want to add "$LBINDIR" to your "'$PATH'
-	elif [ $CLI != "$LBINDIR/$PROJECT_NAME" ]; then
-		fail "An existing $PROJECT_NAME was found at $CLI. Please prepend "$LBINDIR" to your "'$PATH'" or remove the existing one."
+	else
+		# Convert to resolved, absolute paths before comparison
+		CLI_REALPATH="$(cd -- "$(dirname -- "$CLI")" && pwd -P)"
+		LBINDIR_REALPATH="$(cd -- "$LBINDIR" && pwd -P)"
+		if [ "$CLI_REALPATH" != "$LBINDIR_REALPATH" ]; then
+			echo "An existing $PROJECT_NAME was found at $CLI. Please prepend "$LBINDIR" to your "'$PATH'" or remove the existing one."
+		fi
 	fi
 
 	set -e