github: Run update before installing packages
github: Run update before installing packages

The Github CI runners sometimes need to update their apt cache, or
packages fail to install:

 E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/g/gcc-9-cross/gcc-9-powerpc64le-linux-gnu_9.3.0-17ubuntu1~20.04cross2_amd64.deb  404  Not Found [IP: 40.81.13.82 80]
 E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
 Error: Process completed with exit code 100.

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>

shenki/skiboot,shenki/skiboot,shenki/skiboot,shenki/skiboot,shenki/skiboot