fix(tvix/eval): use top-level span for force_with_output

When forcing thunks in `force_with_output`, the call stack of the VM
is actually empty (as the calls are synthetic and no longer part of
the evaluation of the top-level expression).

This means that Tvix crashed when constructing error spans for the
`fallible` macro, as the assumption of there being an enclosing span
was violated.

To work around this, we instead pass the span for the whole top-level
expression to force_for_output and set this as the span for the
enclosing error chain. Existing output logic will already avoid
printing the entire expression as an error span.

This fixes b/213.

Change-Id: I93978e0deaf5bcb0f47a6fa95b3f5bebef5bad4c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7052
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/tests/tvix_tests/eval-fail-builtins-thunk-error.nix

@@ -0,0 +1 @@
+builtins.genList (_: {}.foo) 1

tvix/eval/src/tests/tvix_tests/eval-fail-deep-forced-thunk-error.nix

@@ -0,0 +1 @@
+[ (throw "error!") ]