ci: bump GIT_DATE to 15 months

Currently we're using git clone --since=$GIT_DATE + git checkout $SHA
This causes problems when the $SHA is outside the time period.

Technically one can do git fetch origin $SHA, yet it requires a toggle
in git server-side. There is no obvious way to enable that in gitlab.

Alternatively we can (and should) update piglit and deqp on somewhat
regular intervals - just like we do for Mesa.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

ci/Dockerfile

@@ -10,7 +10,7 @@ ENV CC="gcc-7"
 ENV CXX="g++-7"
 ENV CFLAGS="-g3"
 ENV CXXFLAGS="-g3"
-ENV GIT_DATE="`date +%Y-%m-%d -d \"1 year ago\"`"
+ENV GIT_DATE="`date +%Y-%m-%d -d \"15 months ago\"`"
 ENV MESA_DEBUG=1