Navigator: close the LMDB.Environment on finalize(_:_:_:) (#646)

Ensure that we close the LMDB environment on the exit from
`finalize(_:_:_:)` method.  This is important for platforms which do not
permit alteration of directories with open references.  Having the
environment be closed after finalization enables the generated index to
be moved into the final destination on Windows.

Sources/SwiftDocC/Indexing/Navigator/NavigatorIndex.swift

@@ -1097,7 +1097,9 @@ extension NavigatorIndex {
                     return
                 }
             }
-
+
+            defer { environment.close() }
+
             let database: LMDB.Database
             if let alreadyDefinedDatabase = navigatorIndex.database {
                 database = alreadyDefinedDatabase