Follow-up from "Port EE changes for mirrors/_instructions.html.haml"

mirrors/_instructions.html.haml is incorrectly externalizing a string with variables. We should be using sprintf for mixing the minutes.

Perhaps we should also add this to the PO-linter.

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The following discussion from !21107 (merged) should be addressed:

• @reprazent started a discussion:

  This cannot be picked up by the gettext extractor, we should use sprintf for mixing in variables. So this sentence should look like:

  _("The update action will time out after %{import_timeout_minutes} minutes. For big repositories, use a clone/push combination.") % { import_timeout_minutes: import_will_timeout_message(Gitlab.config.gitlab_shell.git_timeout) }

  I'll create a follow up issue

/cc @godfat