[python3] Enable parallel builds; use vcpkg_install_msbuild()

[ras0219]

See Title

[PhoebeHui]

/azp run

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[strega-nil]

@ras0219 could you merge with master?

[strega-nil]

/azp run

[azure-pipelines]

Pull request contains merge conflicts.

[strega-nil]

/azp run

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[ras0219]

After further work on this, it seems that /p:CL_MPCount is not effective.

However, I think that using the CL environment variable is appropriate for this specific case for two reasons:

1. It is common practice elsewhere to specify flags via environment variables (CXXFLAGS)
2. This flag is a CL-specific flag -- if some other compiler was to be used inside msbuild, it is desirable for it to be ignored. Therefore, the CL environment variable functions perfectly for this use.

While testing, I found two additional significant performance issues:

1. file(COPY) is ~3x slower than xcopy for duplicating the source tree.
2. Spawning ~20 powershell.exe processes takes a long time during vcpkg_copy_tool_dependencies()

After addressing each of these points, the reported time for vcpkg build python3:x64-windows on my machine is down to 38.25s from 1.331mins -- ~2x faster.

[BillyONeal]

However, I think that using the CL environment variable is appropriate for this specific case for two reasons:

1. It is common practice elsewhere to specify flags via environment variables (CXXFLAGS)

I would object much less to CL rather than _CL_. The former I've seen used in build systems sometimes, particularly make derived ones. The latter is the escape hatch for the former.

[ras0219-msft]

/azp run

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[BillyONeal]

/azp run

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[BillyONeal]

Dropping 'reviewed' since it's been changed a lot since then

[ras0219]

This is blocked on fixes in #13019 and #13126

[ras0219-msft]

/azp run

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[ras0219-msft]

/azp run

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[PhoebeHui]

/azp run

[azure-pipelines]

Pull request contains merge conflicts.