environment: Always use public CAS for toolchain

Always use the public CAS instance for toolchain downloads.

Bug: b/233808334
Change-Id: I2770252b90703a1260db08c9ff0716a8784085dd
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/110070
Reviewed-by: Ted Pudlik <tpudlik@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>
diff --git a/recipe_modules/environment/api.py b/recipe_modules/environment/api.py
index 8c82d9f..f4ccc78 100644
--- a/recipe_modules/environment/api.py
+++ b/recipe_modules/environment/api.py
@@ -252,11 +252,14 @@
                     self.m.cipd.ensure(toolchain_dir, pkgs)
 
                 elif toolchain_props['source'] in 'isolated':
-                    self.m.cas.download(
-                        'download',
-                        digest=toolchain_props['version'],
-                        output_dir=toolchain_dir,
-                    )
+                    with self.m.cas.with_instance(
+                        'projects/chromium-swarm/instances/default_instance'
+                    ):
+                        self.m.cas.download(
+                            'download',
+                            digest=toolchain_props['version'],
+                            output_dir=toolchain_dir,
+                        )
 
                 else:  # pragma: no cover
                     raise KeyError(