scripts/oe-publish-sdk: Disable git gc to avoid build errors

I've puzzled over why I see local eSDK test failures. It is due to having a
messy git tree locally, git then runs git gc in the background which races
against the deletion code for the publish test. Disable git gc to avoid this.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>