feat(setup-magento)!: change extension working dir to _ghamagento

[digitalrisedorset]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added
• Docs have been added / updated

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?

In extension mode, setup-magento uses:

../magento2

as the Magento working directory.

This differs from upstream Graycore behavior and may cause issues with interactions involving cache-magento.

Fixes: N/A

What is the new behavior?

Ports parity change from:

graycoreio/github-actions-magento2@a729f8b

The extension mode working directory is now changed to:

_ghamagento

This aligns Mage-OS behavior with upstream Graycore implementation.

Does this PR introduce a breaking change?

• Yes
• No

Consumers relying on the previous hardcoded path:

../magento2

may need to update their workflows.

Consumers using:

steps.setup-magento.outputs.path

should remain unaffected.

Other information

This is a direct parity port from upstream Graycore implementation.

[damienwebdev]

lgtm