meson: Remove redundant env set operation

This is immediately overriden 4 lines later, and starting with Meso
0.50.1 this will lead to a noisy warning.
12 jobs for fix-test-env-setting in 43 minutes and 46 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #235174
gst indent

00:00:46

passed #235173
manifest

00:00:36

 
  Build
passed #235175
build fedora x86_64

00:07:36

passed #235176
build nodebug fedora x86_64

00:04:29

 
  Test
passed #235177
check fedora

00:03:30

passed #235178
integration testsuites fedora

00:11:22

 
  Full Builds
passed #235180
build cerbero android universal

00:11:41

passed #235181
build cerbero cross win32

00:07:56

passed #235182
build cerbero cross win64

00:05:53

passed #235179
build cerbero fedora x86_64

00:06:26

 
  Full Tests
passed #235183
valgrind bad

00:06:23

 
  Apps
passed #235184
android universal examples

00:05:55