[mod] oscar: fix the sourcemap URL in *.min.css

Close https://github.com/searx/searx/issues/2670

Note: clean-css contains a bug:
* a multiline comment or URL adds "$stdin" to the sourcemap (see src/less/logicodev/search.less)
* in this case when the user opens the devtools, the browser fails to load this "https://.../$stdin" URL
* it is not a problem and the error appears only when the user actively tries to debug the CSS.
* seems related to https://github.com/jakubpawlowicz/clean-css/issues/593