fix(publish): stop test_util from requiring a crates.io version

[CraZySacX]

Problem

Commit 9fd334c hoisted test_util, vergen, vergen-gix, and vergen-lib into [workspace.dependencies] in the root Cargo.toml. The test_util entry included a version:

test_util = { version = "10.0.3", path = "test_util" }

Every crate's dev-dependency on test_util inherits workspace = true, so it also inherited that version. That forces cargo to verify a test_util v10.0.3 exists on crates.io during packaging — but test_util is an internal test-helper crate that has never been published. This broke cargo package/cargo publish for vergen-lib, vergen, vergen-git2, vergen-gitcl, and vergen-gix:

error: failed to prepare local package for uploading

Caused by:
  no matching package named `test_util` found
  location searched: crates.io index
  required by package `vergen-lib v10.0.3 (...)`

Separately, test_util/Cargo.toml declared readme = "README.md", but that file has never existed, which independently blocked packaging test_util directly.

Fix

• Root Cargo.toml: drop version from the test_util workspace-dependency entry, keeping only path (matches its pre-hoist, unversioned-path-dep shape, which cargo strips from published manifests).
• test_util/Cargo.toml: replace readme = "README.md" with publish = false, making its internal-only status explicit/enforced and dropping the dangling readme reference.

vergen, vergen-lib, and vergen-gix stay hoisted in [workspace.dependencies] — they weren't actually broken; their --dry-run failures were just "version 10.0.3 not yet published," which self-resolves once published in dependency order.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.21%. Comparing base (9fd334c) to head (eaf29d0).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
+ Coverage   98.02%   98.21%   +0.18%     
==========================================
  Files          25       25              
  Lines        6492     6492              
==========================================
+ Hits         6364     6376      +12     
+ Misses        128      116      -12     

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