refactor(ci): use docker backend + agent placement for process-updates/weekly

Match the build-all cleanup: only the docker backend is used and arch placement
is via the agent label, so the kubernetes backend_options blocks (with arch
nodeSelector and tolerations) are dead. Convert them to docker resource blocks.
weekly-audit had no agent label (it relied on the nodeSelector), so add
agent: ${AGENT} with an AGENT column (artemis=amd64, gaia=arm64) per matrix row.

.crow/process-updates.yaml

@@ -202,7 +202,7 @@ steps:
           /opt/R/$R_VERSION/bin/R -q -e "library(bincraft); upload_package_index(codename = '${OS_ID}', r_minor = '$RMINOR', s3_endpoint = 'https://s3.eu-central-003.backblazeb2.com', s3_region = 'eu-central-003', s3_bucket = 'devxy-rpkgs-binaries', s3_access_key_id = Sys.getenv('B2_S3_ACCESS_KEY'), s3_secret_access_key = Sys.getenv('B2_S3_SECRET_KEY'))" || true
         done
     backend_options:
-      kubernetes:
+      docker:
         resources:
           requests:
             memory: 5Gi
@@ -210,13 +210,6 @@ steps:
           limits:
             memory: 18Gi
             cpu: 3000m
-        nodeSelector:
-          kubernetes.io/arch: ${ARCH}
-        tolerations:
-          - key: 'CI'
-            operator: 'Equal'
-            value: 'true'
-            effect: 'NoSchedule'
 
   - name: Purge CDN cache
     image: reg.devxy.io/docker.io/library/alpine:3.24