ci: specify which zig version to download

.tangled/workflows/build.yaml

···
 steps:
     - name: "Install zig"
       command: "./zig/download.sh"
+      environment:
+          ZIG_VERSION: "master"
     - name: "Build ReleaseFast"
       command: "./zig/zig build -Doptimize=ReleaseFast --summary all"
 

zig/download.sh

···
 #!/bin/bash
 
+if [ -d "zig/$ZIG_VERSION" ]; then
+  echo "Zig $ZIG_VERSION is already installed."
+  exit 0
+fi
+
 JSON=$(curl -s https://ziglang.org/download/index.json)
 
 # Determine the architecture:
···
 
 ZIG_TARGET="$ZIG_ARCH-$ZIG_OS"
 
-URL=$(echo "$JSON" | jq -r ".master.\"$ZIG_TARGET\".tarball")
-EXPECTED_SHA=$(echo "$JSON" | jq -r ".master.\"$ZIG_TARGET\".shasum")
+URL=$(echo "$JSON" | jq -r ".$ZIG_VERSION.\"$ZIG_TARGET\".tarball")
+EXPECTED_SHA=$(echo "$JSON" | jq -r ".$ZIG_VERSION.\"$ZIG_TARGET\".shasum")
 
 curl -O "$URL"
 
···
   exit 1
 fi
 
-if [ ! -d "zig/master" ]; then
-  mkdir zig/master
+if [ ! -d "zig/$ZIG_VERSION" ]; then
+  mkdir zig/$ZIG_VERSION
 fi
 
 CWD=$(pwd)