Symlink all SDK files with go_local_sdk (#3696)

This is required with Go 1.21.1 as the `go` tool reads the `go.env` at
the root of the SDK. Instead of explicitly symlinking that file, symlink
all files except for the ones that already exist, e.g. because they were
added by another SDK rule wrapped with `go_wrap_sdk`.

go/private/sdk.bzl

@@ -473,8 +473,10 @@ def _remote_sdk(ctx, urls, strip_prefix, sha256):
         )
 
 def _local_sdk(ctx, path):
-    for entry in ["src", "pkg", "bin", "lib", "misc"]:
-        ctx.symlink(path + "/" + entry, entry)
+    for entry in ctx.path(path).readdir():
+        if ctx.path(entry.basename).exists:
+            continue
+        ctx.symlink(entry, entry.basename)
 
 def _sdk_build_file(ctx, platform, version, experiments):
     ctx.file("ROOT")