- fix Nix build on macOS Big Sur (by updating to nixpkgs-unstable)
- update gcc-arm-embedded from 9.x to 10.x
- update nix to 2.3.10
- update Alpine Linux to 3.12.3
- allow to change arch in Docker
1. make it work on macOS hosts
2. add quoting to paths to prevent failures if user was foolish
enough to put the repo on a path with spaces
3. use -o pipefail for robustness
4. use wget --no-config to prevent surprises from ~/.wgetrc
5. add --init to docker run for better signals
6. detect effective USER:GROUP like a grown up
7. break fs download url into multiple configurable parts
8. make the script independent on launch location
9. build: generate nix-shell scripts with HEREDOC
9.1. this is easier on eyes, one could review generated script in ./build
9.2. we don't have to escape quotes and chain commands with &&
9.4. added some extra quoting for user-specified values
