gst-env: Don't set DYLD_LIBRARY_PATH on macOS

gst-env: Don't set DYLD_LIBRARY_PATH on macOS

This is not actually needed because everything we build is using
@rpath already, and setting it causes dynamic linker path priority
issues with macOS internals causing *all* programs to fail to run
inside gst-env:

```
$ vim
dyld: Symbol not found: __cg_jpeg_resync_to_restart
  Referenced from: /System/Library/Frameworks/ImageIO.framework/Versions/A/Resources/libTIFF.dylib
  Expected in: /Users/nirbheek/projects/repos/gst-build/_build_macos/subprojects/libjpeg-turbo-2.1.0/libJPEG.dylib
 in /System/Library/Frameworks/ImageIO.framework/Versions/A/Resources/libTIFF.dylib
Abort trap: 6
```

In this case it is caused by libjpeg.dylib, but it can happen with
other dylibs that conflict with dylibs used by macOS internally.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-build/-/merge_requests/257>