ci: fetch vault from artifactapi instead of dnf install

CI installed vault by shelling out to `dnf install vault -y`, which reads
metadata for every enabled repo (appstream/baseos/crb/epel/ha) and downloads
the 169MB vendored vault RPM from the unkin repo on every pipeline run
(~39s per plan/apply job).

- Replace the dnf install with a pinned curl of the upstream vault zip from
  the artifactapi hashicorp-releases remote proxy, extracted with python3 to
  /usr/local/bin/vault.
- Pin the version via a VAULT_VERSION env var (1.20.0).

.woodpecker/plan.yaml

@@ -6,8 +6,9 @@ steps:
     image: git.unkin.net/unkin/almalinux9-opentofu:20260606
     environment:
       VAULT_AUTH_METHOD: kubernetes
+      VAULT_VERSION: "1.20.0"
     commands:
-      - dnf install vault -y
+      - curl -fsSL -o /tmp/vault.zip "https://artifactapi.k8s.syd1.au.unkin.net/api/v1/remote/hashicorp-releases/vault/${VAULT_VERSION}/vault_${VAULT_VERSION}_linux_amd64.zip" && python3 -m zipfile -e /tmp/vault.zip /tmp/ && install -m0755 /tmp/vault /usr/local/bin/vault && rm -f /tmp/vault.zip /tmp/vault /tmp/LICENSE.txt
       - make plan
     backend_options:
       kubernetes: