Bootstrap improvements

[YuliiaKovalova]

Fixes #10300

Context

The PR introduces a simplified and transparent way for getting sdk bits and patching the changes to them using install-scripts.
Details can be found in the doc: https://github.com/dotnet/msbuild/blob/5852b3d778362105203ac4e550fd9456028627c5/documentation/wiki/Bootstrap.md

Notes

• Redundant equality changes were introduced for fixing build errors: https://github.com/dotnet/msbuild/runs/26720044934
• Need to preserve coping of _NuGetRuntimeDependencies, because these dlls are required by bootstrap*\sdk*\NuGet.RestoreEx.targets. Otherwise NuGet.Build.Tasks.dll can not be found : https://dev.azure.com/dnceng-public/public/_build/results?buildId=721344&view=logs&j=20125c81-7edb-551b-693a-61efae016b74&t=7ce25265-aabe-59d3-d73c-06619cfcc3c4&l=158

Might resolve #6566

[JanKrivanek]

This feels as it deserves some very small doc on what is the current bootstrap approach on grabbing bits (does it use '.dotnet'?) and what are other alternatives and their pros and cons.
This change looks OK - I'm just lost if that's the only option and if we're OK with possible introduced full build slowdown.

[JanKrivanek]

This looks great!

I'm putting 'requesting changes' just so that the following two things are reviewed (and I'm fine if consciously dismissed):

• The new task might be better to inherit from some of our ready-made base
• The versioning info might benefit from centralization - if easy to do so. Or/and documenting where it needs to be updated and how is the new version determined

[JanKrivanek]

Looks great!

I have couple comments for consideration

[rainersigwald]

Ok so I am going to be a hyper-nitpicker in the hopes that this shares some of my "how to think about build process changes" opinions.

On my machine, this took 17 seconds (rm -r artifacts && build.cmd), and increased overall build time by like 40%.

I think we can do better!

In general you reduce overall build time by getting work off the critical path. As currently written this is well on the critical path:

1. There's a new task that must be built before the Bootstrap project can proceed.
2. Execution of that task takes a long time.
3. Test projects depend on the bootstrap (because UnitTests.Shared depends on it).

But that's not set in stone.

We can use a RoslynCodeTaskFactory to avoid the project. Then executing the task need have no dependencies, it could be literally the first thing the build does: 9be76bc.

Then we can rearrange the call to the task to be earlier, instead of just-in-time for bootstrap creation (which implicitly delays it until most of the repo has built): d0c2057.

On my machine that knocks a good 15 seconds off the end-to-end build time.

[rainersigwald]

I still don't like that we seem to need to specify an MSBuild version in the post-bootstrap build, but we can attack that later.