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
diff --git a/build_test.bash b/build_test.bash
index b039285..b6d00e2 100755
--- a/build_test.bash
+++ b/build_test.bash
@@ -50,7 +50,9 @@
 
 echo
 echo "Free disk space:"
-df -h
+# Ignore df errors because it errors out on gvfsd file systems
+# but still displays most of the useful info we need
+df -h || true
 
 echo
 echo "Running Bazel smoke test..."