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 <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Ahelenia Ziemiańska <[email protected]>
Signed-off-by: szubersk <[email protected]>
Closes #13066 

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