r271938 added a flatpak_env.json file to forward some variables to webkit-bwrap after flatpak 1.10 changes.

This is working fine most of the time, but if flatpak-spawn is used to invoke a command from the host into the flatpak again (e.g. calling coredumpctl when a layout test fails), the new webkit-flatpak invocation may overwrite the file and delete it when exiting.

This could cause the finally block in the first flatpak process to fail with:

Traceback (most recent call last):
  File "./Tools/Scripts/run-webkit-tests", line 42, in <module>
    flatpakutils.run_in_sandbox_if_available(sys.argv)
  File "./Tools/flatpak/flatpakutils.py", line 1124, in run_in_sandbox_if_available
    sys.exit(flatpak_runner.run_in_sandbox(*app_args))
  File "./Tools/flatpak/flatpakutils.py", line 877, in run_in_sandbox
    os.remove(env_file)
OSError: [Errno 2] No such file or directory: '/home/lauro/dev/WebKit/WebKitBuild/flatpak-env.json'