"Reproduce" action: Support flakes

No more need for a reproduction script! It just says something like

If you have Nix installed, you can reproduce this build on your own machine by running the following command:

nix build github:edolstra/dwarffs/09c823e977946668b63ad6c88ed358b48220f124:hydraJobs.build.x86_64-linux

Created by  Eelco Dolstra  on May 10, 2019
3LEGFJFPEUE7RLBS2GYULCKIOPGI3UNGC6FYGAF7F2HZ2TDHKVUAC
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