Switch to lightweight runner for build

Using a heavyweight runner is quicker even; but not by a significant
enough margin to warrent using one. So use a lightweight runner for the
build

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>

.gitlab-ci.yml

@@ -6,7 +6,7 @@ build:
   stage: build
   image: rust:slim
   tags:
-    - heavyweight
+    - lightweight
   before_script:
     - apt update && apt install -y libssl-dev pkg-config
   script: