Backtick-escape the facter env command in powershell. Fixes issue 3958

When a puppet.facter block is defined on windows, the puppet apply is succeeding but the ps1 task vagrant runs returns 1.

I think this is because the env value is not being quoted, ie:

works:
cmd /c powershell.exe `$env:FACTER_environment='.'.; puppet apply

fails:
PS C:\tmp> cmd /c powershell.exe "$env:FACTER_environment='.'; puppet apply"
The term '.=.' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:6

    .='.' <<<< ; puppet apply
        CategoryInfo : ObjectNotFound: (.=.:String) [], CommandNotFoundException
        FullyQualifiedErrorId : CommandNotFoundException

If i backtick-escape the $env, it works.