fix(ci): give each xvfb-run its own display (#230)

Every `process-cran-updates-ubuntu-2204-arm64` cron run since pipeline 13744 has failed with `xvfb-run: error: Xvfb failed to start` (e.g. 13829).

## Cause

The newer-R loop added in #216 runs two `xvfb-run` calls back to back: the R-minor-sensitive `process_cran_updates` pass, then `local/build-newer-r.R`.
Without `-a`, `xvfb-run` always uses display `:99`, and on exit it kills its Xvfb without waiting for it.
The second call can therefore start while the old server still holds the `:99` lock, so the new Xvfb refuses to start.
Only ubuntu-2204-arm64 hits this because it is the only image with a second R minor installed, so the loop body only runs there.

## Change

- Default `XVFB_ARGS` to `-a` (`--auto-servernum`) in `process-updates.yaml`, `build-all-versions.yaml` and `weekly-rebuild-missing.yaml`, so each call picks a free display.
- The RHEL 10 `xwfb-run` branch still overrides `XVFB_ARGS` with `-c weston`, so it is unchanged.

## Verification

`crow lint .crow/` passes.
A local repro was not possible (`docker run` hangs on the dev host), so the next ubuntu-2204-arm64 cron run is the real check.

Reviewed-on: #230

.crow/weekly-rebuild-missing.yaml

@@ -388,7 +388,7 @@ steps:
       - rm -rf /mnt/cache/R-pkgs/00LOCK-*
       - /opt/R/$R_VERSION/bin/Rscript local/install-bincraft.R
       - /opt/R/$R_VERSION/bin/R -q -e 'packageVersion("bincraft")'
-      - XVFB=$(command -v xwfb-run 2>/dev/null || command -v xvfb-run); XVFB_ARGS=""; if command -v xwfb-run >/dev/null 2>&1; then dnf install -y -q weston 2>/dev/null; XVFB_ARGS="-c weston"; fi
+      - XVFB=$(command -v xwfb-run 2>/dev/null || command -v xvfb-run); XVFB_ARGS="-a"; if command -v xwfb-run >/dev/null 2>&1; then dnf install -y -q weston 2>/dev/null; XVFB_ARGS="-c weston"; fi
       - UVR_R_BIN=/opt/R/$R_VERSION/bin/R local/uvr-install.sh httr2
       - /opt/R/$R_VERSION/bin/R -q -e 'source("local/fetch-rebuild-packages-from-issue.R")'
       - $XVFB $XVFB_ARGS -n $SPLIT_INDEX -- /opt/R/$R_VERSION/bin/Rscript local/rebuild-missing.R $SPLIT_INTO $SPLIT_INDEX $REBUILD_BUDGET_HOURS 2>&1