fix some Tilt hiccups (#5013)

## Which issue(s) this PR closes

- Tilt would complain, on a completely fresh setup, if the
`./dev/helm-local.dev.yml` file didn't exist
- fix issue when building the `grpcio` wheel

Tiltfile

@@ -66,9 +66,21 @@ docker_build_sub(
 
 
 def load_oncall_helm():
-    helm_oncall_values = ["./dev/helm-local.yml"]
-    helm_oncall_values += ["./.github/helm-ci.yml"] if is_ci else ["./dev/helm-local.dev.yml"]
-    yaml = helm("helm/oncall", name=HELM_PREFIX, values=helm_oncall_values, set=twilio_values, namespace="default")
+    helm_oncall_values_files = ["./dev/helm-local.yml"]
+    local_dev_helm_values_file = "./dev/helm-local.dev.yml"
+
+    if is_ci:
+        helm_oncall_values_files.append("./.github/helm-ci.yml")
+    elif os.path.exists(local_dev_helm_values_file):
+        helm_oncall_values_files.append(local_dev_helm_values_file)
+
+    yaml = helm(
+        "helm/oncall",
+        name=HELM_PREFIX,
+        values=helm_oncall_values_files,
+        set=twilio_values,
+        namespace="default",
+    )
     k8s_yaml(yaml)
 
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