Update action-validator in Workflow validation workflow

[motjuste]

The current version of action-validator being used is too old to incorporate more updates to the workflow schema. For example, it failed in PR #1524 due to a false positive that was probably fixed in version 0.5.4.

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CHECKBOX-1601.

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[motjuste]

Fixed in #1526