* google-cloud-sdk: 150.0.0 -> 151.0.0
- gce/create-gce.sh: rewrite using nix-shell shebang and bash
- allows to run the script without being the same directory
- nix-shell install google-cloud-sdk
- some shellcheck cleanups and scripting best practice
- gce/create-gce.sh: do not clobber NIX_PATH: this allows NIX_PATH to be overwritten to build a different release
- gce/create-gce.sh: remove legacy hydra option
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