maintainers: Add script to patch version/sha256 in .nix files

Adds a script to help automatically upgrading packages: this one can
patch name/version attributes like:
    version = "50.1.0";
    name = "bc-1.06";
... to the given version, and updates the sha256 hash to match.

Usage is:

update-source-version <attr> <new-version> [<new-source-hash>]

where:
    - attr is the attribute path of the package
    - new-version is the version string to be patched in
    - new-source-hash is the optional sha256/etc. hash of the source.
      If not given, the script will automatically calculate it.

This is added to a subdirectory where other useful scripts can be added
in the future, like figuring out the newest version from a git repo or
GitHub releases etc.