feat(setup-magento): mkdir app/etc to allow prevent composer plugin crashing

[digitalrisedorset]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/mage-os/github-actions/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (not applicable)
• Docs have been added / updated (not applicable)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

After composer create-project --no-install, the Magento directory structure may not yet contain app/etc.
Some composer plugin/install flows expect this directory to already exist, which can lead to failures during setup workflows.

Fixes: N/A

What is the new behavior?

Adds a defensive filesystem initialization step to ensure app/etc exists immediately after project creation.
This restores parity with the upstream Graycore implementation and improves setup workflow reliability for extension mode installs.

Does this PR introduce a breaking change?

• Yes
• No

Other information

This change intentionally mirrors the existing safeguard present in the Graycore implementation of setup-magento.

[damienwebdev]

lgtm