Make it easier to parse rules_go's .bzl files with Go Starlark (#4140)

**What type of PR is this?**

Feature

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

I'm doing some experiments, where I'm trying to parse rules_go's source
code with Go Starlark so that I can analyze it. In the process I've
noticed that there are some small constructs that Go Starlark doesn't
seem to like. In both those cases It looks like Go Starlark works as
intended.

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

n/a

**Other notes for review**

go/private/rules/wrappers.bzl

@@ -60,7 +60,7 @@ def go_binary_macro(name, **kwargs):
                 # behaviour, so we forbid this.
                 fail("Cannot use select for go_binary with goos/goarch set, but {} was a select".format(key))
 
-    if kwargs.get("linkmode", default = LINKMODE_NORMAL) in LINKMODES_EXECUTABLE:
+    if kwargs.get("linkmode", LINKMODE_NORMAL) in LINKMODES_EXECUTABLE:
         go_binary(name = name, **kwargs)
     else:
         go_non_executable_binary(name = name, **kwargs)