Merge pull request #228297 from amjoseph-nixpkgs/pr/release-cross/comment

pkgs/top-level/release-cross.nix: comment explaining how to build all bootstrapFiles

Changed files
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pkgs
top-level
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pkgs/top-level/release-cross.nix
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e.g.
$ nix-build pkgs/top-level/release-cross.nix -A crossMingw32.nixUnstable --arg supportedSystems '[builtins.currentSystem]'
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To build all of the bootstrapFiles bundles on every enabled platform, use:
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$ nix-build --expr 'with import ./pkgs/top-level/release-cross.nix {supportedSystems = [builtins.currentSystem];}; builtins.mapAttrs (k: v: v.build) bootstrapTools'
*/
{ # The platforms *from* which we cross compile.