fix: properly load non-js config files in genCacheConfig

[LinusBorg]

a2bc927#diff-04612bf191a2c1a06ac248910d9e9415R162 attempted to fix a problem with cache invalidation when javascript config files (like babel.config.js) change their content i.e. depending on an env variable.

The fix was:

          if (absolutePath.endsWith('.js')) {
            // should evaluate config scripts to reflect environment variable changes
            try {
              return JSON.stringify(require(absolutePath))
            } catch (e) {
              return fs.readFileSync(absolutePath, 'utf-8')
            }
         }

However we missed to add an else branch to keep including non-js config files (like .browserslistrc) in the generation of the cache key.

This PR adds that else { branch

close #3631