Make build.zig files compatible with newly released zig 0.12

Fixes #1912

Author: jedisct1

Diff for: fiat-zig/build.zig

@@ -2,7 +2,7 @@ const std = @import("std");
 
 pub fn build(b: *std.Build) void {
     const main_tests = b.addTest(.{
-        .root_source_file = .{ .path = "inversion.zig" },
+        .root_source_file = b.path("inversion.zig"),
     });
     const run_main_tests = b.addRunArtifact(main_tests);
     const test_step = b.step("test", "Run tests");