test: Don't pull the scratch Docker image
test: Don't pull the scratch Docker image

The scratch image can no longer be pulled:
https://github.com/docker/docker/issues/10737

This was originally added as building many images at once in CI was
failing with a claim that the scratch image did not exist. Given it is
no longer an image which gets downloaded, there is no need to pull it.

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

supermario/flynn,kgrz/flynn,kgrz/flynn,kgrz/flynn,tonicbupt/flynn,ozum/flynn,GrimDerp/flynn,justintung/flynn,lmars/flynn,flynn/flynn,supermario/flynn,flynn/flynn,schatt/flynn,benbjohnson/flynn,ozum/flynn,jzila/flynn,Brandywine2161/flynn,josephglanville/flynn,technosophos/flynn,schatt/flynn,tonicbupt/flynn,felixrieseberg/flynn,pkdevbox/flynn,benbjohnson/flynn,arekkas/flynn,ozum/flynn,Brandywine2161/flynn,shads196770/flynn,arekkas/flynn,lmars/flynn,kgrz/flynn,arekkas/flynn,GrimDerp/flynn,rikur/flynn,whouses/flynn,benbjohnson/flynn,felixrieseberg/flynn,clifff/flynn,Brandywine2161/flynn,whouses/flynn,rikur/flynn,shads196770/flynn,rikur/flynn,shads196770/flynn,schatt/flynn,tonicbupt/flynn,tonicbupt/flynn,clifff/flynn,TribeMedia/flynn,schatt/flynn,lmars/flynn,justintung/flynn,TribeMedia/flynn,GrimDerp/flynn,ozum/flynn,shads196770/flynn,pkdevbox/flynn,supermario/flynn,Brandywine2161/flynn,Brandywine2161/flynn,ozum/flynn,jzila/flynn,josephglanville/flynn,shads196770/flynn,justintung/flynn,justintung/flynn,arekkas/flynn,clifff/flynn,clifff/flynn,arekkas/flynn,jzila/flynn,TribeMedia/flynn,kgrz/flynn,supermario/flynn,justintung/flynn,jzila/flynn,tonicbupt/flynn,felixrieseberg/flynn,philiplb/flynn,kgrz/flynn,josephglanville/flynn,philiplb/flynn,rikur/flynn,josephglanville/flynn,technosophos/flynn,flynn/flynn,whouses/flynn,schatt/flynn,technosophos/flynn,pkdevbox/flynn,GrimDerp/flynn,benbjohnson/flynn,pkdevbox/flynn,justintung/flynn,jzila/flynn,benbjohnson/flynn,TribeMedia/flynn,lmars/flynn,flynn/flynn,felixrieseberg/flynn,supermario/flynn,GrimDerp/flynn,TribeMedia/flynn,technosophos/flynn,technosophos/flynn,philiplb/flynn,TribeMedia/flynn,josephglanville/flynn,clifff/flynn,schatt/flynn,lmars/flynn,clifff/flynn,rikur/flynn,pkdevbox/flynn,philiplb/flynn,supermario/flynn,GrimDerp/flynn,flynn/flynn,philiplb/flynn,whouses/flynn,jzila/flynn,whouses/flynn,benbjohnson/flynn,Brandywine2161/flynn,rikur/flynn,shads196770/flynn,josephglanville/flynn,whouses/flynn,ozum/flynn,pkdevbox/flynn,felixrieseberg/flynn