evergreen on windows tests the wrong mongo version

Clear any previous MongoDB installation.

Author: ajdavis

.mci.yml

@@ -267,6 +267,7 @@ functions:
         mv mongodb* mongodb
         chmod +x ./mongodb/bin/mongod${extension}
         if [ ${windows|false} = true ]; then
+            rm -rf /cygdrive/c/mongodb
             cp -r mongodb /cygdrive/c/mongodb
         fi