meson: Allow CUDA_PATH fallback on linux

That's what we've supported via autotools build

Fixes: gstreamer/gst-plugins-bad#966
12 jobs for linux-cuda-path in 42 minutes and 43 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #288034
gst indent

00:00:37

passed #288033
manifest

00:00:46

 
  Build
passed #288035
build fedora x86_64

00:05:13

passed #288036
build nodebug fedora x86_64

00:04:37

 
  Test
passed #288037
check fedora

00:04:31

passed #288038
integration testsuites fedora

00:10:49

 
  Full Builds
passed #288040
build cerbero android universal

00:10:32

passed #288041
build cerbero cross win32

00:08:27

passed #288042
build cerbero cross win64

00:06:22

passed #288039
build cerbero fedora x86_64

00:07:08

 
  Full Tests
passed #288043
valgrind bad

00:05:24

 
  Apps
passed #288044
android universal examples

00:09:56