Use /var/tmp for $TMPDIR for vfs conformance jobs

The conformance tests use the default temporary directory for storing
content when running, and the vfs driver easily needs more than the size
of the tmpfs we have on /tmp when we're running conformance tests.

Signed-off-by: Nalin Dahyabhai <[email protected]>

.cirrus.yml

@@ -188,6 +188,7 @@ conformance_task:
     matrix:
         - env:
             STORAGE_DRIVER: 'vfs'
+            TMPDIR: '/var/tmp'
         - env:
             STORAGE_DRIVER: 'overlay'