google-cloud-sdk: add meta.mainProgram

[kira-bruneau]

Motivation for this change

Adds meta.mainProgram so gcloud can be launched directly with nix run nixpkgs#google-cloud-sdk.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

@iAmMrinal0 @pradyuman @stephenmw @zimbatm

[zimbatm]

I didn't know about the option. Here is a reference to it: NixOS/nix#4498