Build .drone.yml with jsonnet instead of drone

The Drone tool completely ignores the environment key, because of course
it does. It seems the CLI tool has been abandoned for feature parity
here in favour of the built-in enterprise jsonnet parser, so guess we're
doing this.

.drone.yml

@@ -1,5 +1,5 @@
-[
-   {
+---
+{
    "environment": {
       "CACHIX_CACHE": "xeals",
       "CACHIX_SIGNING_KEY": {
@@ -31,8 +31,9 @@
       }
    ],
    "type": "docker"
-   },
-   {
+}
+---
+{
    "environment": {
       "CACHIX_CACHE": "xeals",
       "CACHIX_SIGNING_KEY": {
@@ -64,8 +65,9 @@
       }
    ],
    "type": "docker"
-   },
-   {
+}
+---
+{
    "environment": {
       "CACHIX_CACHE": "xeals",
       "CACHIX_SIGNING_KEY": {
@@ -97,5 +99,5 @@
       }
    ],
    "type": "docker"
-   }
-]
+}
+...