SdkService/BuildPrebuilts: Expect to run in chroot

I just merged a proto change (https://crrev.com/c/4866772) that will
cause this endpoint to always run inside the chroot. Let's make it start
acting like it.

BUG=b:300321083
TEST=./run_tests
TEST=call the endpoint from outside the chroot

Change-Id: Ib66d865afdc8596a415557e4ae651aeff9ad37ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4866950
Tested-by: Greg Edelston <gredelston@google.com>
Reviewed-by: Anuj Jamwal <anujjamwal@google.com>
Commit-Queue: Greg Edelston <gredelston@google.com>
diff --git a/api/controller/sdk.py b/api/controller/sdk.py
index 8417dfa..31e5af2 100644
--- a/api/controller/sdk.py
+++ b/api/controller/sdk.py
@@ -270,9 +270,14 @@
 @faux.all_empty
 @validate.validation_complete
 def BuildPrebuilts(input_proto, _output_proto, _config):
-    """Build the binary packages that comprise the Chromium OS SDK."""
-    chroot = controller_util.ParseChroot(input_proto.chroot)
-    sdk.BuildPrebuilts(chroot, board=input_proto.build_target.name)
+    """Build the binary packages that comprise the Chromium OS SDK.
+
+    Raises:
+        cros_build_lib.Die: If called from outside the chroot. The RPC proto
+            definition guarantees that it should run inside.
+    """
+    cros_build_lib.AssertInsideChroot()
+    sdk.BuildPrebuilts(board=input_proto.build_target.name)
 
 
 @faux.success(_BinhostCLs)