Fix passing environment in startProcess (win)

Previous implementation used newWideCString, which doesn't
handle strings with \0 characters.

tests/osproc/passenv.nim

@@ -0,0 +1,32 @@
+discard """
+  file: "passenv.nim"
+  output: "123"
+  targets: "c c++ objc"
+"""
+
+import osproc, os, strtabs
+
+# Checks that the environment is passed correctly in startProcess
+# To do that launches a copy of itself with a new environment.
+
+if paramCount() == 0:
+  # Parent process
+
+  let env = newStringTable()
+  env["A"] = "1"
+  env["B"] = "2"
+  env["C"] = "3"
+
+  let p = startProcess(
+    getAppFilename(),
+    args = @["child"],
+    env = env,
+    options = {poStdErrToStdOut, poUsePath, poParentStreams}
+  )
+
+  discard p.waitForExit
+
+else:
+  # Child process
+  # should output "123"
+  echo getEnv("A") & getEnv("B") & getEnv("C")