build(deps): bump cachix/install-nix-action from 16 to 20

[dependabot[bot]]

Bumps cachix/install-nix-action from 16 to 20.

Release notes

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

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

install-nix-action-v18

• Fix nodejs warnings & simplify maintenance
• Use python3 when determining number of cores
• Collapse installer log messages

install-nix-action-v17

• curl for downloading the installer is now in quiet mode (since all the bugs have been addressed)
• ensure user profile comes before default profile in PATH
• support https://github.com/actions-runner-controller/actions-runner-controller by using sudo in more places

Commits

• 29bd929 Merge pull request #163 from cachix/fix-nix-2.14
• 167742c don't insist $USER needs to be set
• 3755e30 Support new Nix 2.14 profile PATH
• 193b0d8 Merge pull request #159 from cachix/update-readme
• cb6121c Update README for v19
• 5c11eae Merge pull request #158 from cachix/fix-test
• 846213d update tests
• bd6084e Merge pull request #156 from InternetUnexplorer/update-readme
• 67388ca Merge pull request #157 from zimbatm/default-token
• a4b8aaf Fix rate-limits by using github.token by default
• Additional commits viewable in compare view

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