Update arduino-cli binary filename

[per1234]

The arduino-cli binary filename was recently changed, which broke CI builds:

mv: cannot stat ‘arduino-cli-*-linux64’: No such file or directory

[per1234]

From Slack discussion, it sounds like http://downloads.arduino.cc/arduino-cli/arduino-cli-latest-linux64.tar.bz2 will no longer provide the latest version of arduino-cli and so a further fix needs to be made to restore the previous behavior of the Travis CI build.

I'm waiting on the discussion to settle to make sure I understand the correct action to take and then I will update this PR accordingly.

[per1234]

An update on the "latest" URL situation. From arduino/arduino-cli#346 (comment) it sounds like the plan is to eventually restore the functionality of the URL to point to the latest build again. So it's unclear to me whether it's necessary or desirable to switch from the current system of downloading from the "latest" URL to using install.sh to install the latest version of arduino-cli.

Since the CI build is currently broken due to the file name change which was the original intent of this PR to fix, and the fact that the outdated version of arduino-cli currently provided by "latest" URL doesn't cause any problems for the CI build, I'm going to go ahead and remove the "WIP" status of this PR. If it is necessary to change from using the "latest" URL to install arduino-cli, I'll do that in a dedicated PR. I'd welcome any input on that decision.