Fix compilation of nimscriptapi.nim

To compile the nimscriptapi.nim file, it is needed to import the os
module, but if so the getParams procedure cannot be executed from
nimscript any more. The problem is resolved by conditionally importing
the module.

Related to #680

src/nimblepkg/nimscriptapi.nim

@@ -6,6 +6,9 @@
 import system except getCommand, setCommand, switch, `--`
 import strformat, strutils, tables
 
+when not defined(nimscript):
+  import os
+
 var
   packageName* = ""    ## Set this to the package name. It
                        ## is usually not required to do that, nims' filename is