If the connection fails in a way that might be a certificate error, and
verification was enabled by the new default, mention environment
variable `CMAKE_TLS_VERIFY` in the diagnostic to help users that were
relying on the old behavior turn off server certificate verification in
their environment.
Fixes: #23608
CMAKE_TLS_VERIFY, CMAKE_TLS_CAINFO, CMAKE_NETRC and
CMAKE_NETRC_FILE are now passed through to the underlying
ExternalProject sub-build. Previously, they were silently ignored.
Fixes: #22144
Brad King