ci: avoid "No space left on device" error

Remove unneeded files from the root filesystem to avoid a "No space
left on device" error. A similar trick was committed to optee_os for
the same reason [1].

Link: https://github.com/OP-TEE/optee_os/commit/fa1950059f4128a9b58558d4970c153595463ff3

 [1]
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@foss.st.com>