ci: run "apt-get update" before "apt-get install"

The "sparse" workflow runs "apt-get install" to pick up a few necessary packages. But it needs to run "apt-get update" first, or it risks trying to download an old package version that no longer exists. And in fact this happens now, with output like: 2021-07-26T17:40:51.2551880Z E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/c/curl/libcurl4-openssl-dev_7.68.0-1ubuntu2.5_amd64.deb 404 Not Found [IP: 52.147.219.192 80] 2021-07-26T17:40:51.2554304Z E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing? Our other ci jobs don't suffer from this; they rely on scripts in ci/, and ci/install-dependencies does the appropriate "apt-get update". Signed-off-by: Jeff King <peff@peff.net> Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>