build: Switch to envsubst for templating

.gitea/workflows/actions.yaml

@@ -57,10 +57,7 @@ jobs:
           apt-get install -y \
             genisoimage
 
-          sed 's|__ROOT_HASHED_PASSWORD__|${{ secrets.ROOT_HASHED_PASSWORD }}|g' \
-            cloud-init/user-data.template > cloud-init/user-data
-          sed 's|__PUBLIC_SSHKEY__|${{ secrets.PUBLIC_SSHKEY }}|g' \
-            cloud-init/user-data.template > cloud-init/user-data
+          envsubst < cloud-init/user-data.template > cloud-init/user-data
 
           genisoimage \
             -output seed.iso \
@@ -68,6 +65,9 @@ jobs:
             -joliet \
             -rock \
             cloud-init/meta-data cloud-init/user-data
+        env:
+          ROOT_HASHED_PASSWORD: ${{ secrets.ROOT_HASHED_PASSWORD }}
+          PUBLIC_SSHKEY: ${{ secrets.PUBLIC_SSHKEY }}
 
   # semrel:
   #   name: Semantic Release