Upgrade to the new actions API (#65)

In future Bazel releases ctx.file_action will be removed.
ctx.action.write should be used instead.

With this change tests pass even if --incompatible_new_actions_api
is used.

Bazel issue:
bazelbuild/bazel#5825

lib/unittest.bzl

@@ -166,10 +166,10 @@ def _end(env):
         "EOF",
         "exit %d" % len(env.failures),
     ])
-    env.ctx.file_action(
+    env.ctx.actions.write(
         output = env.ctx.outputs.executable,
         content = cmd,
-        executable = True,
+        is_executable = True,
     )
 
 def _fail(env, msg):