Enable long path support in rnw-dependencies

[asklar]

When an enlistment clone is created in a deep part of the filesystem, sometimes we can get build breaks if the paths we deal with are >MAX_PATH.
Enabling long paths should palliate that.

https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file#enable-long-paths-in-windows-10-version-1607-and-later

[JunielKatarn]

Sample command-line workaround (pending #5012):

msbuild.exe Microsoft.ReactNative.sln /p:RootOutDir=D:\short

Outputting the target artifacts to a shorter root path allows keeping the cloned sources in place for now.