Expand Linux test matrix

[thejohnfreeman]

This change makes progress on the goal of #4371. It does not replicate the full matrix implemented in #3851, but it does replicate the 1.ii section of the Linux matrix. It leverages our "heavy" self-hosted runners and my Docker image for a rippled build environment, and demonstrates what I believe is a simple, readable pattern for future matrices.

[shichengripple001]

looks good to me

[intelliot]

notes:

• flaky tests occur on Mac; is there an equivalent of heavy?
  • no. per @shichengripple001 , we may get a Mac mini and use it for our self-hosted runners.
• this currently removes the Mac job. @thejohnfreeman will add a workflow for Mac.
  • even if it is flaky, we would rather have that, than no job at all.

[drlongle]

Looks good to me.

[legleux]

Maybe push a new ubuntu 18.04 image with GCC-10?

[thejohnfreeman]

Why 18.04?

[legleux]

So we can test it into perpetuity?

[thejohnfreeman]

Ok. I think there's a case to be made to pick one configuration to test on multiple versions of Ubuntu. I don't have an 18.04 image with everything available right now. Won't take me long to make one, but perhaps we can punt that to a future PR?

[thejohnfreeman]

I think this is ready to merge with this message:

This change makes progress on the plan in #4371. It does not replicate the full [matrix] implemented in #3851, but it does replicate the 1.ii section of the Linux matrix. It leverages our "heavy" self-hosted runners, and demonstrates a repeatable pattern for future matrices.

[matrix]: https://github.com/XRPLF/rippled/blob/d794a0f3f161bb30c74881172fc38f763d7d46e8/.github/README.md#continuous-integration