Add support for non-nix users

Allow non-nix users to build proost with cargo build, without specifying +nightly or rustup default nightly

assigned to @pigeonmoelleux

created branch 79-add-support-for-non-nix-users to address this issue

mentioned in merge request !68 (merged)

added To review label

removed To review label

closed with merge request !68 (merged)

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