fix(ci): export SHORT_SHA so yq env() can read it

yq env(SHORT_SHA) on lines 330 and 339 requires SHORT_SHA as an
environment variable, not just a shell variable. Without export, yq
receives an empty value and the Update Infra Image Tags job fails on
every merge to main.

Regression from GRO-311 fix (commit 0d610f5).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci.yml

@@ -314,7 +314,7 @@ jobs:
           if [ -z "$TAG" ]; then
             TAG="$(date -u +%Y.%m.%d)-${GITHUB_SHA::7}"
           fi
-          SHORT_SHA="${SHA::7}"
+          export SHORT_SHA="${SHA::7}"
           echo "Updating dev overlay image tags to: $TAG"
           echo "Updating migration/seed Job names with SHA: $SHORT_SHA"
           cd /tmp/infra