chore: bump spec test version to v1.6.1

Upgrades from v1.6.0-beta.2 to v1.6.1. Changes between versions:
- Fulu fork epoch and blob schedule (#4689)
- Gossip attestation committee_bits fix (#4742)
- Various Gloas spec cleanups (#4719-#4741)
- SSZ Union integer encoding fix (#4725)

None of these affect our state_transition or SSZ test vectors.

🤖 Generated with AI assistance

Author: lodekeeper-z

build.zig

@@ -25,7 +25,7 @@ pub fn build(b: *std.Build) void {
     const options_spec_test_options = b.addOptions();
     const option_spec_test_url = b.option([]const u8, "spec_test_url", "") orelse "https://github.com/ethereum/consensus-specs";
     options_spec_test_options.addOption([]const u8, "spec_test_url", option_spec_test_url);
-    const option_spec_test_version = b.option([]const u8, "spec_test_version", "") orelse "v1.6.0-beta.2";
+    const option_spec_test_version = b.option([]const u8, "spec_test_version", "") orelse "v1.6.1";
     options_spec_test_options.addOption([]const u8, "spec_test_version", option_spec_test_version);
     const option_spec_test_out_dir = b.option([]const u8, "spec_test_out_dir", "") orelse "test/spec/spec_tests";
     options_spec_test_options.addOption([]const u8, "spec_test_out_dir", option_spec_test_out_dir);