refactor(ci): route workflows by agent group label instead of named agent

Replace the per-agent `agent: ${AGENT}` label with a group label
`group: rpkgs-${ARCH}`, so workflows target the rpkgs-amd64 / rpkgs-arm64
agent pools rather than a specific agent. Drops the now-unused AGENT matrix
variable across all build/process/weekly workflows.

.crow/build-all-versions-install-deps.yaml

@@ -30,14 +30,12 @@ skip_clone: true
 
 labels:
   platform: linux/${ARCH}
-  agent: ${AGENT}
+  group: rpkgs-${ARCH}
 
 matrix:
   include:
     - ARCH: amd64
-      AGENT: artemis
     - ARCH: arm64
-      AGENT: gaia
 
 steps:
   - name: 'Install deps and bincraft'