fixes #12998 nim doc regression (#13117)

koch.nim

@@ -24,7 +24,7 @@ when defined(i386) and defined(windows) and defined(vcc):
   {.link: "icons/koch-i386-windows-vcc.res".}
 
 import
-  os, strutils, parseopt, osproc, streams
+  os, strutils, parseopt, osproc
 
 import tools / kochdocs
 
@@ -45,6 +45,7 @@ Options:
   --help, -h               shows this help and quits
   --latest                 bundle the installers with a bleeding edge Nimble
   --stable                 bundle the installers with a stable Nimble (default)
+  --nim:path               use specified path for nim binary
 Possible Commands:
   boot [options]           bootstraps with given command line options
   distrohelper [bindir]    helper for distro packagers
@@ -605,6 +606,7 @@ when isMainModule:
       case normalize(op.key)
       of "latest": latest = true
       of "stable": latest = false
+      of "nim": nimExe = op.val.absolutePath # absolute so still works with changeDir
       else: showHelp()
     of cmdArgument:
       case normalize(op.key)