source-bundle: --except argument is ignored

Background

[//]: # In the source-bundle command --except argument is ignored

Use case to reproduce the bug

• [ ] Clone https://gitlab.com/baserock/definitions.git
• [ ] Run: bst --colors --option arch armv8l64 source-bundle bootstrap/stage3-sysroot.bst --except gnu-toolchain/stage2.bst
• [ ] tar -tf bootstrap-stage3-sysroot.tar.gz | grep stage2
• [ ] Elements from stage2 are listed in the previous command

Acceptance Criteria

[//]: # When elements are listed in --except they shouldn't be in the final tarball