TF: cmake: use the github mirror

We see connection timeout on cmake.org:

curl: (7) Failed to connect to cmake.org port 443: Connection timed out

Try the github mirror instead.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Change-Id: I438041633c72d83e1f0705de453ac5ee691e7947
2 files changed