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

.github/workflows/scripts/reclaim_disk_space.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+set -eu
+
+# remove 4GiB of images
+sudo systemd-run docker system prune --force --all --volumes
+
+# remove unused software
+sudo systemd-run rm -rf \
+  "$AGENT_TOOLSDIRECTORY" \
+  /opt/* \
+  /usr/local/* \
+  /usr/share/az* \
+  /usr/share/dotnet \
+  /usr/share/gradle* \
+  /usr/share/miniconda \
+  /usr/share/swift \
+  /var/lib/gems \
+  /var/lib/mysql \
+  /var/lib/snapd