Bump cachix/install-nix-action from v10 to v12

Dependencies

• [2] 6WKHHS6H Bump cachix/install-nix-action from v8 to v10 (#791)
• [3] NKPSTN6H add github ci tests
• [4] 27T5ZSQE Only run the 'build' test
• [5] LX326CB7 Add hydra OpenAPI description (#750)

Change contents

• replacement in .github/workflows/test.yml at line 12
  [3.206]→[2.0:42](∅→∅)
  − - uses: cachix/install-nix-action@v10
  [3.206]

  [3.247]
  + - uses: cachix/install-nix-action@v12
• replacement in .github/workflows/test.yml at line 21
  [3.124]→[2.43:85](∅→∅)
  − - uses: cachix/install-nix-action@v10
  [3.124]

  [3.165]
  + - uses: cachix/install-nix-action@v12