Msbuild max cpu

[TheAngryByrd]

Description

Allows properties like MaxCpuCount to be used on unix.

Mostly checking CI to see if it breaks something.

If available, link to an existing issue this PR fixes. For example:

TODO

Feel free to open the PR and ask for help

• [ ] New (API-)documentation for new features exist (Note: API-docs are enough, additional docs are in help/markdown)

• [ ] unit or integration test exists (or short reasoning why it doesn't make sense)

  Note: Consider using the CreateProcess API which can be tested more easily, see https://github.com/fsharp/FAKE/pull/2131/files#diff-4fb4a77e110fbbe8210205dfe022389b for an example (the changes in the DotNet.Testing.NUnit module)

• [ ] boy scout rule: "leave the code behind in a better state than you found it" (fix warnings, obsolete members or code-style in the places you worked in)

• [ ] (if new module) the module has been linked from the "side navigation" menu, edit docs/data.json.

• [ ] (if new module) the module is in the correct namespace.

• [ ] (if new module) the module is added to Fake.sln (dotnet sln Fake.sln add src/app/Fake.*/Fake.*.fsproj)

• [ ] Fake API guideline is honored