fix(plugins): Root folder name in generated .tar.gz

🗒️ Description

Fixes the name of the root folder in the generated .tar.gz file to always be fixtures instead of the name of the output folder.

@pdobacz

None

[x] All: Set appropriate labels for the changes.
[x] All: Considered squashing commits to improve commit history.
[x] All: Added an entry to CHANGELOG.md.
[x] All: Considered updating the online docs in the ./docs/ directory.
[x] Tests: All converted JSON/YML tests from ethereum/tests have been added to converted-ethereum-tests.txt.
[x] Tests: A PR with removal of converted JSON/YML tests from ethereum/tests have been opened.
[x] Tests: Included the type and version of evm t8n tool used to locally execute test cases: e.g., ref with commit hash or geth 1.13.1-stable-3f40e65.
[x] Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.