build(deps): bump cachix/install-nix-action from 18 to 21

[dependabot[bot]]

Bumps cachix/install-nix-action from 18 to 21.

Release notes

Sourced from cachix/install-nix-action's releases.

install-nix-action-v21

• pin Nix to 2.15.1 (recent releases broke too many things)
• fix the action to work on custom containers

install-nix-action-v20

https://discourse.nixos.org/t/nix-2-14-0-released/25900/4Fix support for Nix 2.14 new profile location: NixOS/nix#5226

install-nix-action-v19

• expose github_access_token option and use the default github token by default to prevent rate limiting
• support --no-daemon installation

Commits

• 4b933aa Nix: 2.15.1
• 3580693 Merge pull request #179 from joergdw/fix-action-path
• 3eb7a24 Merge pull request #178 from cachix/docs/149
• 840ed7c Document how to pass env vars to modern nix commands
• b2f4229 Fix action to make it work on custom containers;
• e304541 fix #170
• 3988b72 pin Nix to 2.15.0
• 763a380 Bump revision in README
• 67e9fd7 bump revision in readme
• be4cef7 Merge pull request #166 from l0b0/refactor/linting
• Additional commits viewable in compare view

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