playbooks/system-test: Avoid running out of storage space (part 2)

This should have been part of commit 571dc97.

#1464

Author: debarshiray

.github/workflows/ubuntu-tests.yaml

@@ -146,5 +146,6 @@ jobs:
       - name: System tests
         run: bats --timing test/system/001-version.bats test/system/002-help.bats test/system/108-completion.bats
         env:
+          TMPDIR: /var/tmp
           TOOLBX: /usr/local/bin/toolbox
         working-directory: containers/toolbox