Add Swarming link for RPi disk space alert.
Add Swarming link for RPi disk space alert.

Change-Id: I028446aec2217900fbf57c36cedf06e44f064145
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/215441
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Auto-Submit: Ben Wagner aka dogben <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot