CI: remove unused packages/snaps

Removing portions of packages/snaps directly with rm can result in
unexpected errors when running `apt update`.  Free up the additional
space by removing (some) packages with the proper tools.

This change frees up slightly less space than before, but it is
expected to still be sufficient.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14374

.github/workflows/scripts/reclaim_disk_space.sh

@@ -1,20 +1,19 @@
-#!/bin/sh
+#!/bin/sh -x
 
 set -eu
 
 # remove 4GiB of images
 sudo systemd-run docker system prune --force --all --volumes
 
+# remove unused packages
+sudo apt remove -q --purge firefox
+
 # 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 \
+  /usr/share/swift
-  /var/lib/gems \
-  /var/lib/mysql \
-  /var/lib/snapd