SignTool: Use .NET Core MSBuild for Non-Windows

[ellahathaway]

Closes #14430

I tested these changes locally by running the unit tests via ./test.sh. For e2e testing, I initially considered configuring a local project to pull the SignTool package from a local feed. While this seems viable, the setup can be tedious. If there's a better method for e2e testing, please let me know.

[mmitche]

My favorite method of end 2 end local testing has always been to do the local feed dance. Key points:

• Arcade repo with changes I build with /p:OfficialBuildId=Date.Rev.
• Test repo I clone separately
• Update arcade package in global.json for each test iteration

For actual validation for this, we should augment arcade-validation to:

• Run -sign in non-Windows PR legs (dry run)

#14431 would cover enabling real signing there.

[ellahathaway]

My favorite method of end 2 end local testing has always been to do the local feed dance. Key points:

• Arcade repo with changes I build with /p:OfficialBuildId=Date.Rev.
• Test repo I clone separately
• Update arcade package in global.json for each test iteration

For actual validation for this, we should augment arcade-validation to:

• Run -sign in non-Windows PR legs (dry run)

#14431 would cover enabling real signing there.

Awesome thanks for explaining. I'll work on e2e testing and update this PR if changes are needed.

[ellahathaway]

Finished local testing on linux. Build ran as expected and I verified via the binlogs that the tool was using .NET Core msbuild for building MicroBuild.