This is caused by the fact that you do not have our certificate authority (Starfield Technologies) installed for use by your curl, which drives the communication with Git.

A quick workaround is to run:

git config --global http.sslVerify false

or prepend to your Git command:

 env GIT_SSL_NO_VERIFY=true git clone [your URL]

The former permanently changes your local Git settings; the latter changes it only for that specific command.

 If you wish to install the root certificate, you can obtain it from Starfield directly. Instructions for installation depend on your operating system.