AFAICS, the test_embed failure first appeared after GH-29041 (bpo-45582) was merged. After GH-29041, there has been numerous fixes for test_embed:

    $ git log --oneline | grep bpo-45582
    3f398a77d3 bpo-45582: Fix test_embed failure during a PGO build on Windows (GH-30014)
    b0b3086279 bpo-45582: Write empty pybuilddir.txt on Windows to allow relocatable build directories (GH-29979)
    06c4ae8b13 bpo-45582: Fix framework path and bootstrap build (GH-29954)
    b7ef27bc08 bpo-45582: Ensure PYTHONHOME still overrides detected build prefixes (GH-29948)
    f16f93e527 bpo-45582: framework build: modPath must not be const (GH-29944)
    af1db4eb55 bpo-45582: Fix getpath_isxfile() and test_embed on Windows (GH-29930)
    628abe4463 bpo-45582: Fix signature of _Py_Get_Getpath_CodeObject (GH-29921)
    0ae4e0c959 bpo-45582 Fix prototype of _Py_Get_Getpath_CodeObject. (GH-29907)
    7d7c91a8e8 bpo-45582: Add a NOT operator to the condition in getpath_isxfile (GH-29906)
    ccb73a0d50 bpo-45582: Fix out-of-tree build issues with new getpath (GH-29902)
    99fcf15052 bpo-45582: Port getpath[p].c to Python (GH-29041)