Pick up QTC_PLUGIN_DIRS from environment

Setting QTC_PLUGIN_DIRS as an environment variable is easier
to manage than passing it every time explicitly to e.g. the qmake
call. It also is in line with other environment variables used in
Qt Creator, like WITH_LLDB, QT_BREAKPAD_ROOT_PATH etc.

Change-Id: Ief94aaf7c2a16ee62dcbf27c9f217496b1f74ad5
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>