[ci] Use ArchLinux as base.

1 job for v1.3.0 in 33 seconds (queued for 6 seconds)
Status Name Job ID Coverage
  Test
failed package #21670

00:00:33

 
Name Stage Failure
failed
package Test
  Problem 1
- Root composer.json requires PHP extension ext-iconv * but it is missing from your system. Install or enable PHP's iconv extension.

To enable extensions, verify that they are enabled in your .ini files:
- /etc/php7/php.ini
You can also run `php --ini` in a terminal to see which files are used by PHP in CLI mode.
Alternatively, you can run Composer with `--ignore-platform-req=ext-iconv` to temporarily ignore these required extensions.
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1