Remove unused files from GitHub runners

Majority of the software installed by default in GitHub runners is
irrelevant to OpenZFS. Reclaimed space could be used for more/bigger
vdev files. File deletion happens in the background, leveraging
systemd-run - the workflow is not significantly slowed down.

Before

Filesystem      Size  Used Avail Use% Mounted on
/dev/root        84G   53G   31G  63% /

After

Filesystem      Size  Used Avail Use% Mounted on
/dev/root        84G   15G   70G  18% /

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #13066