Cirrus-CI: add df/du to track down disk full issues

Cirrus-CI: add df/du to track down disk full issues

Cirrus-CI: show mounted filesystems and free space at start

CI runs have been encountering disk full errors.  Add a `df` invocation
so that we can see what we're working with.

(cherry picked from commit af11c2029006b64da41a78f2e2dcc910e13ce07a)

Cirrus-CI: Add more information to help track down disk full issues

Execute df(1) before and after the build (reporting in MiB for
consistency), and du(1) of /usr/obj.  Also include the uname.

(cherry picked from commit f2f24008a2b1b3c7a1c4b75e9b9c2d0f08abc27b)