Specify $HOME instead of ~ in skiabot-slave-start-on-boot.sh
Specify $HOME instead of ~ in skiabot-slave-start-on-boot.sh

Motivation: This script created a ~ directory when $SKIA_REPO_DIR was not specified.

BUG=skia:

Review URL: https://codereview.chromium.org/1584063004

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