Re-base the nix-installer.sh against rustup's .sh#1639

grahamc wants to merge 3 commits intomainDeterminateSystems/nix-installer:mainfrom

graham/fh-990-last-week-we-saw-the-determinate-nix-installer-hung-in-ciDeterminateSystems/nix-installer:graham/fh-990-last-week-we-saw-the-determinate-nix-installer-hung-in-ciCopy head branch name to clipboard