phase1: workaround gitverify 1st build failures on fresh workers

phase1: workaround gitverify 1st build failures on fresh workers

Currently every 1st build on ephemeral build worker results in the
failure of the `gitverify` build step as the stock buildbot Git() build
step for some reasons just clones the Git repository into detached HEAD
state.

On the 2nd build using the same build worker Git() checkouts the branch
and thus makes `gitverify` step happy and the builds then continues
normally.

This needs to be fixed properly, either by adjusting the `gitverify`
check or adding suitable `mode` into Git() build step, but this needs
more time and testing.

So for now, lets simply workaround that issue by running Git() step two
times.

References: #5
Signed-off-by: Petr Štetiar <ynezz@true.cz>