fix(satp-hermes): add nix support

[oGranny]

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Fixes #4035

added flake.nix file that defines a Nix flake for setting up all required toolchains and updated the README.md file with instructions for using Nix as an alternative to global toolchain installation.

[RafaelAPB]

@oGranny thank you for your contribution! Could you add to the description why this functionality is useful? Would you please add a test asserting that Nix installation and setup works (ci, local)?