Remove duplicate environment variables from CI

.drone.yml

@@ -2,11 +2,12 @@ kind: pipeline
 type: docker
 name: continuous integration
 
+environment:
+  ZIG_VERSION: 0.13.0
+
 steps:
   - name: setup
     image: ubuntu:latest
-    environment:
-      ZIG_VERSION: 0.13.0
     commands:
       - apt-get update
       - apt-get install -y xz-utils wget libsdl2-dev glslang-tools spirv-tools
@@ -17,16 +18,12 @@ steps:
 
   - name: build
     image: ubuntu:latest
-    environment:
-      ZIG_VERSION: 0.13.0
     commands:
       - export PATH=$PATH:$(pwd)/zig-linux-x86_64-$${ZIG_VERSION}
       - zig build
 
   - name: test
     image: ubuntu:latest
-    environment:
-      ZIG_VERSION: 0.13.0
     commands:
       - export PATH=$PATH:$(pwd)/zig-linux-x86_64-$${ZIG_VERSION}
       - zig build test