[rc2] Allow the build host to be copied from M.CA.Workspaces.MSBuild

[AndriySvyryd]

Fixes #35638

Description
Microsoft.CodeAnalysis.Workspaces.MSBuild looks for the build host in the user's project, but it's not present as by default content is not transitively used from NuGet packages. This PR fixes that and adds more diagnostic information on MSBuild workspace failures. MSBuildLocator usage is also removed as it's no longer needed with the current version of Microsoft.CodeAnalysis.Workspaces.MSBuild.

Customer impact
Customers using Microsoft.EntityFrameworkCore.Tasks to precompile queries will get an exception on build/publish. The only workaround is to use other tools instead.

How found
Found during validation.

Regression
Yes, from EF 9.0.x

Testing
Tested manually.

Risk
Low, the code change only affects the broken scenario.

[artl93]

RC2. core scenario. I am curious if the the lack of the build parameter would be considered a breaking change if the build now works differently or could fail in CI if provided without the command line being supported?

[AndriySvyryd]

I am curious if the lack of the build parameter would be considered a breaking change if the build now works differently or could fail in CI if provided without the command line being supported?

If the user uses Microsoft.CodeAnalysis.Workspaces.MSBuild directly that might be the case, but if it's just through the tasks that we provide it shouldn't be any different compared to EF 9.