Fix version info missing from native binaries

[dagood]

For https://github.com/dotnet/core-setup/issues/7635. To be ported to 3.0: it's an infra regression.

Allow the SDK to restore SourceLink and find its MSBuild props and targets files. These are used to fetch Git information.

Use Arcade-generated _version.c. The new _version.c is identical to the old version.cpp. The host build.sh generated an empty version.cpp when it doesn't already exist at the specified path, which caused the "No version information" message to be included.

[dagood]

For reference, on Windows:

#> (ls .\artifacts\bin\win-x64.Debug\corehost\*.dll).VersionInfo.ProductVersion
42,42,42,42424 @Commit: f35878b35101b653bef9f3dd308b314d1bf481b7
42,42,42,42424 @Commit: f35878b35101b653bef9f3dd308b314d1bf481b7
42,42,42,42424 @Commit: f35878b35101b653bef9f3dd308b314d1bf481b7
42,42,42,42424 @Commit: f35878b35101b653bef9f3dd308b314d1bf481b7
42,42,42,42424 @Commit: f35878b35101b653bef9f3dd308b314d1bf481b7
42,42,42,42424 @Commit: f35878b35101b653bef9f3dd308b314d1bf481b7

On Linux:

#> strings artifacts/bin/linux-x64.Debug/corehost/* | grep '@('
@(#)Version 42.42.42.42424 @Commit: 42e0645857c2d0e96103412c9c304dc4ce867252
@(#)Version 42.42.42.42424 @Commit: 42e0645857c2d0e96103412c9c304dc4ce867252
@(#)Version 42.42.42.42424 @Commit: 42e0645857c2d0e96103412c9c304dc4ce867252
@(#)Version 42.42.42.42424 @Commit: 42e0645857c2d0e96103412c9c304dc4ce867252
@(#)Version 42.42.42.42424 @Commit: 42e0645857c2d0e96103412c9c304dc4ce867252

(42 is Arcade for "dev build", my Linux build commit starting with 42 is a coincidence.)