Keep builders coming when DigitalOcean sells out a size or region (#861)

* Keep builders coming when DigitalOcean sells out a droplet size

ric1 sold out of g5-32vcpu-64gb-50gb, and every create came back 422. curl -f
dropped the reason and set -e ended the tick, so builders only appeared when
capacity happened to free up, and the journal showed nothing but "curl: (22)".

Try each of SIZES in turn, logging DigitalOcean's refusal message, and fail
the tick only when every size is refused. Builders now power off however
start.sh exits, so a failed registration is reaped instead of counting as a
booting runner until MAX_AGE_MINUTES. The controller unit pulls the checkout
before each tick, so merged controller fixes reach the box.

* Create builders in any region that has the size in stock

ric1 sold out of g5-32vcpu-128gb-50gb within minutes of the box switching to
it. Builders need nothing from a particular region, so read DigitalOcean's
size catalog once per tick and try each of SIZES in every region it lists in
stock, REGIONS first if set. A refused pair is dropped for the rest of the
tick.

ci/runner-cloud-init.yaml

@@ -45,6 +45,10 @@ write_files:
     content: |
       #!/bin/bash
       set -euo pipefail
+      # Power off after the one job, and also when the download or the
+      # registration fails: the controller deletes powered-off droplets, but
+      # counts a running one as a runner still booting until MAX_AGE_MINUTES.
+      trap 'sudo poweroff' EXIT
       cd /home/runner
       mkdir -p actions-runner && cd actions-runner
       arch=$(uname -m); [[ $arch == x86_64 ]] && arch=x64
@@ -58,8 +62,6 @@ write_files:
         --labels "__RUNNER_LABELS__" \
         --replace
       ./run.sh
-      # One job done. Power off; the controller deletes powered-off droplets.
-      sudo poweroff
 
 runcmd:
   # With no account ssh key attached, DO expires root's password, and sshd