Feature Request: Allow using multiple distributions for build matrix expansion

[rajat2004]

Hi,
I would like to use the build matrix feature to run jobs on multiple Linux distributions like Bionic, Focal, etc. Currently, each job needs to specified individually, since matrix expansion doesn't work with dist.

Please see https://travis-ci.community/t/using-build-matrix-for-multiple-ubuntu-distributions/9187 for detailed description and discussion.

An example of final config file, expecting to get 2 x 5 jobs -

language: python
os: linux
dist:
  - bionic
  - focal

env:
  - CIINSTALL=yes SCRIPT=12   # BasicSetup + GenSoftware
  - CIINSTALL=yes SCRIPT=3    # ML-Basic
  - CIINSTALL=yes SCRIPT=4    # Build-OpenCV
  - CIINSTALL=yes SCRIPT=14   # BasicSetup + OpenCV (Conda OpenCV)
  - CIINSTALL=yes SCRIPT=5    # BuildML

Current travis.yml - https://github.com/rsnk96/Ubuntu-Setup-Scripts/blob/master/.travis.yml

This is my first time creating an issue here, so sorry for any details missing, etc. I would love to contribute this myself as well, any direction on how to proceed will be very much appreciated!