fix(linux): Fix shared keyboard (un-)installation in Bionic

Ubuntu 18.04 Bionic comes with an older Python version (3.6) that
doesn't support the `capture_output` parameter on `subprocess.run`
which leads to a crash when installing or uninstalling keyboads to
the shared area. This change works around the problem by calling
a different method on Bionic.

Fixes #6564.