Bump cachix/install-nix-action from 16 to 18

[dependabot[bot]]

Bumps cachix/install-nix-action from 16 to 18.

Release notes

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

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

• daddc62 Merge pull request #144 from cachix/fix-ci
• 8500bf7 try to fix act test
• 4024cf0 Merge pull request #143 from ggreif/patch-1
• 3d1155e readme: don't perpetuate old versions
• e17a164 fix #140: python -> python3
• 92d3622 Merge pull request #139 from sigprof/collapse-log-output
• 6c5ba55 install-nix.sh: Collapse log messages
• 451e611 Merge pull request #134 from lovesegfault/composite-action
• 112054f refactor: replace with a simpler composite action
• 24e801e Merge pull request #123 from cachix/dependabot/github_actions/actions/setup-go-3
• Additional commits viewable in compare view

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[dependabot[bot]]

Looks like cachix/install-nix-action is no longer a dependency, so this is no longer needed.