build.zig: use Step.UpdateSourceFiles for zig1.wasm

We were already using this for `stage1/zig.h`, but `stage1/zig1.wasm`
was being modified directly by the `wasm-opt` command. That's a bad idea
because it forces the build system to assume that `wasm-opt` has side
effects, so it is re-run every time you run `zig build update-zig1`,
i.e. it does not interact with the cache system correctly. It is much
better to create non-side-effecting `Run` steps (using `addOutput*Arg`)
where possible so that the build system has a more correct understanding
of the step graph.

Author: mlugg

build.zig

@@ -678,14 +678,14 @@ fn addWasiUpdateStep(b: *std.Build, version: [:0]const u8) !void {
     });
     run_opt.addArtifactArg(exe);
     run_opt.addArg("-o");
-    run_opt.addFileArg(b.path("stage1/zig1.wasm"));
+    const optimized_wasm = run_opt.addOutputFileArg("zig1.wasm");
 
-    const copy_zig_h = b.addUpdateSourceFiles();
-    copy_zig_h.addCopyFileToSource(b.path("lib/zig.h"), "stage1/zig.h");
+    const update_zig1 = b.addUpdateSourceFiles();
+    update_zig1.addCopyFileToSource(optimized_wasm, "stage1/zig1.wasm");
+    update_zig1.addCopyFileToSource(b.path("lib/zig.h"), "stage1/zig.h");
 
     const update_zig1_step = b.step("update-zig1", "Update stage1/zig1.wasm");
-    update_zig1_step.dependOn(&run_opt.step);
-    update_zig1_step.dependOn(&copy_zig_h.step);
+    update_zig1_step.dependOn(&update_zig1.step);
 }
 
 const AddCompilerModOptions = struct {