build: remove nix store paths from releases

[rvolosatovs]

Feature or Problem

• fix nix flake show
• remove the secret-key-base hack, use actual env variables and --impure
• remove /nix/store references from Elixir releases. This one is a fun one, Elixir is used to build the project and then copies itself to the release - we would not be able to build using "patched" Elixir, since /bin/sh etc. does not (and should not) exist in nix sandbox during build, therefore we patch these out as part of the mix build

Related Issues

Release Information

Consumer Impact

Testing

Pull an artifact from a CI run as usual and test in a Docker container, e.g. docker run --rm -it -v$(pwd):$(pwd) -w$(pwd) ubuntu from a directory containing the binary. Run (let it fail) and inspect ~/.local/share/.burrito/*. Currently there's just one shebang with nix store ref, in cgi handler, but we should not use that anyway

Built on platform(s)

• x86_64-linux
• aarch64-linux
• x86_64-darwin
• aarch64-darwin
• x86_64-windows

Tested on platform(s)

• x86_64-linux
• aarch64-linux
• x86_64-darwin
• aarch64-darwin
• x86_64-windows

Unit Test(s)

Acceptance or Integration

Manual Verification

See Testing