Adds the possibility to initialize a fresh VMR off of the current `dotnet/installer` commit.
Similarly to the tarball generation, the process can be triggered by running:
```
./build.sh /p:InitializeVMR=true /p:VmrDir=[Path were VMR will be created]
```
* Initial checkin of source-build tarball build infra
* Add a couple more comments
* Update eng/SourceBuild.Version.Details.xml based on PR review
Co-authored-by: Chris Rummel <crummel@microsoft.com>
* Updates based on PR review comments
Co-authored-by: Chris Rummel <crummel@microsoft.com>
