sh-test-lib: Don't run apt-get update every time

Run apt-get update once per 6 hours, rather than every time dependencies
are installed.

Change-Id: I6eaa15ac47e6a3cc7d60a6ea45fa663183547ca4
Signed-off-by: Dan Rue <dan.rue@linaro.org>
(cherry picked from commit 61c3a6499a253a8e2343d8c407fd75fcf9d16886)
1 file changed