MsvcToolchain: Fix environment detection for some setups

Instead of redirecting the set output to a temporary file output
it on stdout with some begin/end markers. This way we don't need to
write any filenames into the batch file.

Task-number: QTCREATORBUG-11539
Change-Id: I7cfef0f0361d5a2e93fd46298be40f511158b694
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>