Cache toolchains

Signed-off-by: Jacob Kiers <jacob@jacobkiers.net>

.drone.jsonnet

@@ -11,10 +11,16 @@ local getStepName(arch) = 'check ' + arch;
 local buildForArch(arch) = {
   name: getStepName(arch),
   image: 'img.kie.rs/jjkiers/rust-dind-cross:1.62-slim',
-  volumes: [{
+  volumes: [
+    {
       name: 'dockersock',
       path: '/var/run',
-  }],
+    },
+    {
+      name: 'rust-toolchains',
+      path: '/usr/local/rustup/toolchains',
+    },
+  ],
   commands: [
     'echo Hello World from Jsonnet on ' + arch + '!',
     'cross build --release --target ' + arch,
@@ -92,6 +98,12 @@ local check_steps() = [
         path: '/srv/drone/docker-dind-rust',
       },
     },
+    {
+      name: 'rust-toolchains',
+      host: {
+        path: '/srv/drone/rust-toolchains',
+      },
+    },
   ],
 
   image_pull_secrets: ['docker_private_repo'],