BuildElement without install root are disallowed by remote execution build

Summary

Freedesktop SDK has element split-locales.bst of kind manual which does not create the install-root directory but has some integration commands. This fails to build.

This works for local builds, but not remote builds.

Steps to reproduce

Build Freedesktop SDK with remote execution.

What is the current bug behavior?

Build fails with the following error.

[00:00:32][80243ad6][build:split-locales.bst             ] FAILURE Directory '/buildstream-install' was not found inside the sandbox, unable to collect artifact contents

What is the expected correct behavior?

It should behave the same way as without remote execution. That is succeed.

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.4

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