Put manifest generation behind a switch

[chcosta]

Fixes #5255 dotnet/arcade-validation#1409

Puts manifest generation behind a switch (enabled in Arcade), so that I can investigate Linux failures further.

Validated with:

• switch omitted - https://dev.azure.com/dnceng/public/_build/results?buildId=600329&view=results
• switch included - https://dev.azure.com/dnceng/public/_build/results?buildId=600285&view=results

[JohnTortugo]

LGTM

[chcosta]

WTF, CI is failing tests with

It was not possible to find any compatible framework version
The framework 'Microsoft.NETCore.App', version '2.1.0' was not found.
  - The following frameworks were found:
      3.1.1 at [/__w/1/s/.dotnet/shared/Microsoft.NETCore.App]

I don't see how my change could possibly be related. I'm looking... :(

[chcosta]

Ok, I think I found it, this is me

[tmat]

Assuming this is just a workaround and will be reverted?

[chcosta]

Yes, already undone - #5267

[tmat]

Thanks!