Point to correct github ref for ukca

Whyborn · Whyborn · commit 900de99afcf6 · 2026-04-01T11:35:47.000+11:00
diff --git a/spack.yaml b/spack.yaml
@@ -17,6 +17,7 @@ spack:
       - model="vn13p1-am"
       - jules_ref="2026.01.0"
       - um_ref="2026.01.0"
+      - ukca_ref="615a547a060ef80ae6019cf63d409602473e1e68"
 
     # Indirect dependencies
     netcdf-c:
