Kokoro: Log disk utilization in status message

Use 'df' to print the disk utilization whenever we print a task status.
This is to help try to identify spurious out-of-disk flakes we've been seeing.

Change-Id: If26527923e2af720786626a6248699927eb5f430
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/90642
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

infra/kokoro/linux/build.sh

@@ -30,6 +30,12 @@ else
     TMP_DIR=/tmp
 fi
 
+echo "*****************************************************************"
+echo "* build.sh"
+echo "*"
+echo "* df:"
+df
+echo "*****************************************************************"
 
 # --privileged is required for some sanitizer builds, as they seem to require PTRACE privileges
 docker run --rm -i \

infra/kokoro/linux/docker.sh

@@ -50,6 +50,9 @@ function status {
     echo ""
     echo "*****************************************************************"
     echo "* $@"
+    echo "*"
+    echo "* df:"
+    df
     echo "*****************************************************************"
     echo ""
     task_begin $@