Build server can't build with tests at net6.0

[QuintillusCFC]

Currently, the build server fails if the tests use net6.0, likely because its Docker image uses Mono, which imitates net4.7.2.

We should figure out a way to fix this, maybe:

• Add a step to fetch net6.0 and build using that?
• Figure out why the tests need to be on net6.0, and find a way to go back to net4.7.2
• Something else?

[WildWeazel]

@JimOfLeisure had mentioned some interest in this

[QuintillusCFC]

This wound up being a semi-blocker to copying all the assets at once. So, I'm looking at it now, with TheOrioli's solution. Seems to be working. Now trying to DRY out the configuration.