fix #17853 (ascii message separator broke json nim dump) (#17887)

tests/osproc/treadlines.nim

@@ -1,15 +1,18 @@
 discard """
-  output: '''Error: cannot open 'a.nim'\31
+  output: '''
+Error: cannot open 'a.nim'\31
 Error: cannot open 'b.nim'\31
 '''
   targets: "c"
 """
 
 import osproc
+from std/os import getCurrentCompilerExe
 
 var ps: seq[Process] # compile & run 2 progs in parallel
+const nim = getCurrentCompilerExe()
 for prog in ["a", "b"]:
-  ps.add startProcess("nim", "",
+  ps.add startProcess(nim, "",
                       ["r", "--hint[Conf]=off", "--hint[Processing]=off", prog],
                       options = {poUsePath, poDaemon, poStdErrToStdOut})