Ignore errors when running df in build_test.bash

On glinux, df -h prints an "Operation not permitted" error
along with the rest of the output, and exits with status code 1,
causing the rest of the build_test script to not run. Ignore this
error.

Test: Running build_test.bash
Change-Id: I5422d0459ed471671d8d99a883c9ac3fca2e91b8