zig: use "const" for variables that are never mutated (#1742)

Author: jedisct1

fiat-zig/build.zig

@@ -1,7 +1,7 @@
 const std = @import("std");
 
 pub fn build(b: *std.Build) void {
-    var main_tests = b.addTest(.{
+    const main_tests = b.addTest(.{
         .root_source_file = .{ .path = "inversion.zig" },
     });
     const run_main_tests = b.addRunArtifact(main_tests);