Keep order for PATH components (#4252)

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**What type of PR is this?**

> Bug fix

**What does this PR do? Why is it needed?**

The starlark `dict` already will return items in the order in which they
where inserted on iteration. There is no need to explicitly sort the
items. But it does break use cases, where the user has explicitly
defined a order of `PATH` components. As for example is possible with
the `env` attribute of `cc_args()` with the new `rule_cc` based
toolchains.

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

go/private/context.bzl

@@ -890,7 +890,7 @@ def _cgo_context_data_impl(ctx):
     for tool_path in tool_paths:
         tool_dir = tool_path[:tool_path.rfind("/")]
         path_set[tool_dir] = None
-    paths = sorted(path_set.keys())
+    paths = path_set.keys()
     if ctx.configuration.host_path_separator == ":":
         # HACK: ":" is a proxy for a UNIX-like host.
         # The tools returned above may be bash scripts that reference commands