The test added in this commit shows the problem. Previously the
SigningKeyID was set to the RootCert not the local leaf signing cert.
This same bug was fixed in two other places back in 2019, but this last one was
missed.
While fixing this bug I noticed I had the same few lines of code in 3
places, so I extracted a new function for them.
There would be 4 places, but currently the InitializeCA flow sets this
SigningKeyID in a different way, so I've left that alone for now.
