Add HUGO_NUMWORKERMULTIPLIER

And use that to calculate number of workers, if set, else fall back to number of logical CPUs.

Also tweak the relevant related settings to match the new setup.

Also remove the setting of `runtime.GOMAXPROCS` as this has been the default behaviour since Go 1.5.

Fixes #5814

main.go

@@ -14,16 +14,12 @@
 package main
 
 import (
-	"runtime"
-
 	"os"
 
 	"github.com/gohugoio/hugo/commands"
 )
 
 func main() {
-
-	runtime.GOMAXPROCS(runtime.NumCPU())
 	resp := commands.Execute(os.Args[1:])
 
 	if resp.Err != nil {