v4: using font variants crashes on nodejs

E.g., using mathsf throws an error on nodejs:

Can't load './output/svg/fonts/svg/dynamic/sans-serif': No asyncLoad method specified
.../node_modules/mathjax-full/js/util/Retries.js:25
    var err = new Error('MathJax retry');
              ^

Error: MathJax retry
    at retryAfter (.../node_modules/mathjax-full/js/util/Retries.js:25:15)
    at FontData.getChar (.../node_modules/mathjax-full/js/output/common/FontData.js:580:41)

Similarly for \texttt etc.