Add RunsOn configuration for GPU runner support (#438)

Add .github/runs-on.yml with QuantEcon Ubuntu 24.04 AMI configuration.
This file must be on main branch for RunsOn to recognize the custom image.

AMI: ami-0edec81935264b6d3
Region: us-west-2

This is a prerequisite for PR #437 which enables GPU builds.

Author: mmcky

.github/runs-on.yml

@@ -0,0 +1,6 @@
+images:
+  quantecon_ubuntu2404:
+    platform: "linux"
+    arch: "x64"
+    ami: "ami-0edec81935264b6d3"
+    region: "us-west-2"