Trac #34104: Build & Test workflow: Run apt-get update first

https://github.com/sagemath/sagetrac-
mirror/runs/7138537921?check_suite_focus=true
{{{
Get:9 http://archive.ubuntu.com/ubuntu focal-updates/universe amd64
python3.8-venv amd64 3.8.10-0ubuntu1~20.04.4 [5448 B]
Get:10 http://archive.ubuntu.com/ubuntu focal/universe amd64
python3-venv amd64 3.8.2-0ubuntu2 [1228 B]
E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/c/curl/li
bcurl3-gnutls_7.68.0-1ubuntu2.11_amd64.deb  404  Not Found [IP:
91.189.91.38 80]
Fetched 6341 kB in 1s (6155 kB/s)
E: Unable to fetch some archives, maybe run apt-get update or try with
--fix-missing?
}}}

URL: https://trac.sagemath.org/34104
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Frédéric Chapoton

.github/workflows/build.yml

@@ -32,6 +32,7 @@ jobs:
           # Install test tools.
           # Installation of python3-venv can be removed as soon as a
           # base image with a release including #33822 is available
+          apt-get update
           apt-get install -y git python3-venv
           # Reuse built SAGE_LOCAL contained in the Docker image 
           ./bootstrap