Fix CustomPlugin and SpecflowCompatiblity.ReqnrollPlugin builds by adjusting pack

[obligaron]

🤔 What's changed?

• Reqnroll.CustomPlugin uses IncludeBuildOutput to avoid processing empty BuildOutput.
• Reqnroll.SpecFlowCompatibility.ReqnrollPlugin sets IsPackable to false, because it's included in Reqnroll.SpecFlowCompatibility (part of lib nuget folder)

⚡️ What's your motivation?

Fixes #1057

🏷️ What kind of change is this?

• 🐛 Bug fix (non-breaking change which fixes a defect)

♻️ Anything particular you want feedback on?

I checked that the generated nuget packages have the same content as the last published in nuget but it would be good if someone else looks again.

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[clrudolphi]

And what caused nuget packing to fail for those two plugins, now, compared to what we've been doing since the packing approach was (recently) updated? It seems like this came out of nowhere.

[obligaron]

And what caused nuget packing to fail for those two plugins, now, compared to what we've been doing since the packing approach was (recently) updated? It seems like this came out of nowhere.

I have no idea. I could reproduce it locally and fixed it (for my pc). It also looks like it doesn't happen all the time (some CI builds run, some fail).

[gasparnagy]

@obligaron What should we do? Are we confident to merge this or do you want to make further analysis?

[obligaron]

It fixes compilation issues in CI and locally. The nuget packages look the same (to me). I would merge this.

[gasparnagy]

thx