Improve readability of `vmr-source-build-and-validate.yml`

[ellahathaway]

vmr-source-build-and-validate.yml is difficult to read because of all the conditions on the leg.distro parameter. We should look for ways to simplify and improve the readability of this file.

[MichaelSimons]

[Triage] This is a limitation caused by what is supported by AzDO as mentioned in dotnet/dotnet#595 (comment).