Download aarch64 artifact in launcher script

[mkurz]

Follow-up to #1726 / #1714.
Now that scala-ci has aarch64 support since v0.1.19 the launcher script should also download the correct artifact.

This pull request does more or less the same we do in the sbt script here, just since a couple of hours actually 😉 (sbt/sbt#7110)

Currently on my Linux aarch64 machine:

$ ./scala-cli.sh 
Downloading https://github.com/VirtusLab/scala-cli/releases/download/v0.1.19/scala-cli-x86_64-pc-linux.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100 30.6M  100 30.6M    0     0  6166k      0  0:00:05  0:00:05 --:--:-- 8667k

With this pull request:

$ ./scala-cli.sh 
Downloading https://github.com/VirtusLab/scala-cli/releases/download/v0.1.19/scala-cli-aarch64-pc-linux.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100 30.3M  100 30.3M    0     0  8329k      0  0:00:03  0:00:03 --:--:-- 10.0M

[lwronski]

LGTM @mkurz Thanks!