Require task-creating clients to relativize to setup directory

[ranweiler]

Right now, when generating task configs, we prepend certain config parameters with the path prefix setup/ when generating the config. If a client creates a task with target_exe = "my-fuzzer.exe", then config.target_exe == "setup/my-fuzzer.exe". In general, the server expects the client to submit a blob container-relative virtual blob for such config parameters.

Move this responsibility to the client, and require that task creators relativize paths as-needed using the {setup_dir} expansion variable. This expansion will not yet work in general, as described in #2387. As an interim step, have the CLI templates effect the hardcoding that is currently happening server-side.

AB#41312886

[ranweiler]

This is doable, runs into even-worse hardcoding friction in server-side paths where we'd soon need to implement placeholder expansion. See the discussion in #2399.

Closing this for now, since any solution that shifts the burden to the client side really should use variable expansion, and be holistically implemented, which may require more significant changes to the repro VM runtime code.