update CI for zig 0.9.1/use variable for same

.drone.yml

@@ -6,12 +6,6 @@ steps:
 - name: make
   image: alpine:3.15.0
   environment:
-    USERNAME:
-      from_secret: docker_username
-    PASSWORD:
-      from_secret: docker_password
-    REGISTRY:
-      from_secret: docker_registry
     ZIG_VERSION: 0.9.1
   commands:
   # DRONE_STAGE_ARCH is fine, but we can't substitute directly because zig
@@ -22,9 +16,9 @@ steps:
   #
   # DRONE_STAGE_ARCH=amd64
   # DRONE_STAGE_ARCH=arm64
-  - wget https://ziglang.org/download/${ZIG_VERSION}/zig-linux-x86_64-${ZIG_VERSION}.tar.xz
-  - tar x -C /usr/local -f zig-linux-${ZIG_VERSION}.tar.xz
-  - ln -s /usr/local/zig-linux-${ZIG_VERSION}/zig /usr/local/bin/zig
+  - wget https://ziglang.org/download/$${ZIG_VERSION}/zig-linux-x86_64-$${ZIG_VERSION}.tar.xz
+  - tar x -C /usr/local -f zig-linux-$${ZIG_VERSION}.tar.xz
+  - ln -s /usr/local/zig-linux-$${ZIG_VERSION}/zig /usr/local/bin/zig
   - apk add git
   - (cd codegen && zig build test)
   - zig build -Dfetch # implicitly does a codegen