[github-action] bump DeterminateSystems/nix-installer-action from 20 to 21#19

IliyanKostov9 merged 2 commits intomasterZProfile/flutter-client:masterfrom

dependabot/github_actions/DeterminateSystems/nix-installer-action-21ZProfile/flutter-client:dependabot/github_actions/DeterminateSystems/nix-installer-action-21Copy head branch name to clipboard