meson: Do not install cogl config.env if installed tests are disabled

Until meson 0.50, setting the install parameter in 'configure_file' is ignored
if 'install_dir' is set. Then until mutter doesn't depend on such meson version
cogl_installed_tests_libexecdir should be empty unless have_installed_tests is
false, or this file will be installed anyway.

See https://github.com/mesonbuild/meson/issues/4160
2 jobs for master in 6 minutes and 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #205102
build-mutter

00:01:10

 
  Test
passed #205103
test-mutter

00:04:53