Bump cachix/install-nix-action from 12 to 15

[dependabot[bot]]

Bumps cachix/install-nix-action from 12 to 15.

Release notes

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

install-nix-action-v15

• Enable flakes and the new CLI by default
• Fix #97: single installation for Act and Docker should be fixed
• Fix #91: retry official installation as it can sometimes return 400

install-nix-action-v14.1

github actions disable Spotlight by default now, so we don't have to do it in this action anymore

install-nix-action-v14

• Create X number of users where X = 2 * cpu cores
• Linux: fallback to single-user installation if there's no systemd (support for https://github.com/nektos/act)
• Fixes a subtle bug on self-hosted runners when installation fails

install-nix-action-v13

Adds support for install_options that allows passing custom installer options. Run sh <(curl -L https://nixos.org/nix/install) --help to see what's supported.

This feature is useful for installing/testing development versions of Nix.

Commits

• 5f45af0 cleanup
• 489b4a2 Merge pull request #105 from cachix/resilient-install
• b93cdea Merge pull request #109 from cachix/fix-97
• 783053e Resilient installation
• c664ef3 fix #97: install nix config correctly for single-user- mode
• 74ef8ad Merge pull request #108 from cachix/enable-experimental-features-by-default
• 53fb030 enable flakes & nix-command features by default
• e776d82 Merge pull request #107 from cachix/dependabot/github_actions/actions/checkou...
• 98f3210 v14.1
• 8026a8b chore(deps): bump actions/checkout from 2.3.5 to 2.4.0
• Additional commits viewable in compare view

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