Coin: QNX 8.0 QEMU: turn off fs-nfs3 write caching

Tests run from the NFS-mounted build tree, and fs-nfs3 buffers writes
by default. A path stat() can then miss the client's own unflushed
data: tst_QNetworkReply's ioGetFromFile and chaining intermittently
read a short Content-Length on QNX 8.0 (4 of 296 qtbase runs over ten
days, none of 362 on QNX 7.1). A local reproducer shows stale sizes in
3 of 1200 checks by default and none with -w sync=hard.

Pick-to: 6.12 6.11
Task-number: QTBUG-151095
Change-Id: I9fa66b8a8f4be504ab75f8dbec82e6cc9a3aeee8
Reviewed-by: Toni Saario <toni.saario@qt.io>