build: no longer use deprecated api in sass_bundle rule (#16064)

Switches away from deprecated Bazel API that now needs to be enabled
explcitly within Bazel 0.24.0 (`incompatible_disallow_old_style_args_add`)

Author: devversion

tools/sass_bundle.bzl

@@ -7,11 +7,11 @@ def _sass_bundle(ctx):
 
   # The entry-point scss file for the bundle.
   args.add("--entry")
-  args.add(ctx.attr.entry_point.files)
+  args.add(ctx.file.entry_point)
 
   # The list of files that can be included in the bundle.
   args.add("--srcs")
-  args.add(ctx.files.srcs, join_with =",")
+  args.add_joined(ctx.files.srcs, join_with = ",")
 
   # The generated bundle's filename.
   args.add("--output")