You can now optionally invoke update-source-versions with:
* --system flag changing the host platform, to be passed dirrectly to Nix commands.
This is useful for binary packages which have different sources for each platform.
* --file flag allowing to change the file to be modified. This is useful for packages
that offer multiple variants, listed in a different file than the derivation itself;
e.g. packages.nix of Sublime Text 3.
* --version-key, which is now a keyword flag instead of a positional argument.
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