fix(gh-cache/save): Make destination directory(s) before copying files there

[smockle]

When key includes /, gh-cache/save failed with the following error:

cp: cannot create regular file 'foo/bar.json': No such file or directory

This PR fixes the error by creating the destination directory(s) before performing the copy.

I validated this fix:

• https://github.com/github/accessibility-sandbox/actions/runs/20113417672 (Hubber access only) is a workflow run where the directory did not exist (and so was created)
• https://github.com/github/accessibility-sandbox/actions/runs/20113449371 (Hubber access only) is a follow-up workflow run after the directory did exist (and was reused without error)

[Copilot]

Pull request overview

This PR fixes a bug in the GitHub Actions cache save action where copying files to nested directories would fail if the parent directories didn't already exist in the cache branch. The fix adds a mkdir -p command to create the necessary directory structure before attempting the copy operation, following the same pattern already used in the restore action.

Key Changes:

• Added directory creation step before file copy operation to prevent "No such file or directory" errors

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