ci: move input interpolation into an intermediate environment variable

[typfel]

What's new in this PR

best practice according to: https://docs.github.com/en/actions/reference/security/secure-use#use-an-intermediate-environment-variable

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PR Submission Checklist for internal contributors

• The PR Title
  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.