Install Dart Sass binary for GH pages

[jarrodmillman]

We are getting this error:

Error: error building site: render: failed to render pages: render of "page" failed:
"/home/runner/work/numpy.org/numpy.org/themes/scientific-python-hugo-theme/layouts/_default/baseof.html:9:7":
execute of template failed: template: _default/single.html:9:7:
executing "_default/single.html" at <partial "css.html" .>:
error calling partial: "/home/runner/work/numpy.org/numpy.org/themes/scientific-python-hugo-theme/layouts/partials/css.html:36:83":
execute of template failed: template: partials/css.html:36:83:
executing "partials/css.html" at <toCSS $cssOpts>:
error calling toCSS: no Dart Sass binary found in $PATH

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