Detection is broken there, too, as UIOP relies on setting the variable
before dumping the image in its portability wrapper dump-image which we
don't use at all.
Change-Id: If7bea5a8522a2e64707b1ee88d62d420bd00a952
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5112
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>