Bump cachix/install-nix-action from v8 to v10 (#791)

Bumps cachix/install-nix-action from v8 to v10.

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Created by  dependabot[bot]  on July 14, 2020
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