fix windows assistant upgrade glitch

Prevent windows assistant from trying (and failing) to upgrade itself,
which has never been supported on windows.

The new windows build is made with UPGRADE_LOCATION set, which enabled this
code path that had never run on windows before, and doesn't work. I don't
want to try to support self-upgrade on windows, or generally on other OS's
than the ones where its working, so added a check for that. This way the
build can keep setting UPGRADE_LOCATION and if some later git-annex does
learn how to upgrade itself on some OS, it won't need changing the build
setup.

Assistant/WebApp/Configurators/Preferences.hs

@@ -21,7 +21,7 @@ import Annex.NumCopies
 import Utility.DataUnits
 import Git.Config
 import Types.Distribution
-import qualified BuildInfo
+import Assistant.Upgrade
 
 import qualified Data.Text as T
 import qualified System.FilePath.ByteString as P
@@ -59,7 +59,7 @@ prefsAForm d = PrefsForm
 		, ("disabled", NoAutoUpgrade)
 		]
 	autoUpgradeLabel
-		| isJust BuildInfo.upgradelocation = "Auto upgrade"
+		| upgradeSupported = "Auto upgrade"
 		| otherwise = "Auto restart on upgrade"
 
 	positiveIntField = check isPositive intField