CI: apt update before apt install

[olleolleolle]

Fixes build failures foundin #935.

This PR fixes those unrelated failures.

Reading package lists...
Building dependency tree...
Reading state information...
Suggested packages:
  libcurl4-doc libidn11-dev libkrb5-dev libldap2-dev librtmp-dev libssh2-1-dev
The following NEW packages will be installed:
  libcurl4-openssl-dev
0 upgraded, 1 newly installed, 0 to remove and 15 not upgraded.
Need to get 321 kB of archives.
After this operation, 1541 kB of additional disk space will be used.
Ign:1 http://azure.archive.ubuntu.com/ubuntu focal-updates/main amd64 libcurl4-openssl-dev amd64 7.68.0-1ubuntu2.7
Err:1 http://azure.archive.ubuntu.com/ubuntu focal-updates/main amd64 libcurl4-openssl-dev amd64 7.68.0-1ubuntu2.7
  404  Not Found [IP: 52.252.75.106 80]
E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/c/curl/libcurl4-openssl-dev_7.68.0-1ubuntu2.7_amd64.deb  404  Not Found [IP: 52.252.75.106 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
Error: Process completed with exit code 100.