Always use computer domain to bind to RootDSE

[bill-long]

Issue:
If a user is logged in to a machine in the Exchange forest using a user account from a trusted forest, binding to LDAP://RootDSE gives us the user forest.

Fix:
Get the computer's domain and explicitly bind to that RootDSE.

[dpaulson45]

Should probably do the same thing for $rootDSE = [ADSI]("GC://RootDSE") as well.

[lusassl-msft]

I'm wondering if we need to adjust the Test-CVE-2021-34470.ps1 function as well.

$schemaMaster = [System.DirectoryServices.ActiveDirectory.Forest]::GetCurrentForest().SchemaRoleOwner

GetCurrentForest() --> Gets a Forest object for the current user context.
https://learn.microsoft.com/en-us/dotnet/api/system.directoryservices.activedirectory.forest.getcurrentforest?view=netframework-4.7.2#system-directoryservices-activedirectory-forest-getcurrentforest

[bill-long]

Good point. In fact, all calls to GetCurrentForest() need to be changed to GetComputerDomain().Forest.

[lusassl-msft]

Found no other functions that use the problematic code. Tested HealthChecker and it seems to work fine with the updated calls (as the return object should be the same). Approved and ready to merge from my point of view.