bst source-checkout creates an empty checkout if a workspace is open

Summary

When checking out an element's source code using bst source-checkout, the contents of the source-checkout will be empty if the element has an open workspace.

Steps to reproduce

Using doc/examples/autotools as an example:

bst workspace open --directory hello-workspace hello.bst
bst source-checkout hello.bst hello-checkout
find hello-checkout/hello

What is the current bug behavior?

The directory is empty if the workspace is open

What is the expected correct behavior?

The directory should have the exact same contents as if there was not an open workspace (i.e. changes in the workspace are not staged as part of source-checkout)

Relevant logs and/or screenshots

Possible fixes

Other relevant information

The exact same problem happened with source-checkout's precursor, source-bundle

• BuildStream version affected: /milestone %BuildStream_v1.4

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