update CI for zig 0.9.1/use variable for same

.drone.yml

@@ -17,8 +17,8 @@ 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-x86_64-$${ZIG_VERSION}.tar.xz
-  - ln -s /usr/local/zig-linux-x86_64-$${ZIG_VERSION}/zig /usr/local/bin/zig
+  - 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