tests: fix meson test env setup to make sure we use the right gst-plugin-scanner

This is the same fix that was applied in gst-plugins-good in
gstreamer/gst-plugins-good!603
and fixes the testsuite running in gst-build.

Part-of: <gstreamer/gst-editing-services!223>
30 jobs for !223 with backport-mr-219-into-1.18-tests-plugin-scanner-path in 60 minutes and 11 seconds (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #6188260
alpine amd64 manifest builder docker

00:00:21

passed #6188259
android docker

00:00:18

passed #6188257
cerbero fedora amd64 docker

00:00:22

passed #6188256
fedora amd64 docker

00:00:19

passed #6188262
gst-indent amd64 docker

00:00:14

 
  Preparation
passed #6188265
gst indent

00:00:13

passed #6188264
manifest

00:00:16

 
  Build
passed #6188283
gstreamer
build cerbero cross-android universal

00:11:36

passed #6188291
gst-ios-13.2
build cerbero cross-ios universal

00:26:06

passed #6188285
gstreamer
build cerbero cross win32

00:06:30

passed #6188287
gstreamer
build cerbero cross win64

00:05:24

passed #6188281
gstreamer
build cerbero fedora x86_64

00:06:22

passed #6188289
gst-macos-10.15
build cerbero macos x86_64

00:12:32

passed #6188272
build clang fedora x86_64

00:05:37

manual #6188278
docker windows 1809 allowed to fail manual
build msys2
passed #6188267
build nodebug fedora x86_64

00:05:17

passed #6188269
build static fedora x86_64

00:05:13

passed #6188270
build static nodebug fedora x86_64

00:04:15

passed #6188274
docker windows 1809
build vs2017 amd64

00:12:08

passed #6188276
docker windows 1809
build vs2017 x86

00:12:33

 
  Test
passed #6188292
gstreamer
check fedora

00:08:38

passed #6188293
gstreamer
integration testsuites fedora 1/4

00:14:03

passed #6190020
gstreamer
integration testsuites fedora 2/4

00:13:53

passed #6188295
gstreamer
integration testsuites fedora 3/4

00:13:40

passed #6188296
gstreamer
integration testsuites fedora 4/4

00:15:52

passed #6188297
gstreamer
valgrind ges

00:07:05

failed #6188294
gstreamer
integration testsuites fedora 2/4

00:13:33

 
  Integrate
passed #6188298
cross-android universal examples

00:07:08

passed #6188300
gst-ios-13.2
cross-ios universal examples

00:03:37

passed #6188299
documentation

00:14:06