Error when compile mono-unity-2018.3-mbe, no permission to clone mono-build-tools-extra

this problem is just simply like this one:https://forum.unity.com/threads/compiling-mono-branch-2018-2-mbe-failed.636046/
I'm trying to compile https://github.com/Unity-Technologies/mono/tree/unity-2018.3-mbe, for upgrade android application without ILRuntime/xLua since there's too many limits such as "cannot use generic". I need to change a little logic to load dll in mono.

I use it to start compile:
$ ./external/buildscripts/build_runtime_android.sh
Here's those errors:
>>> Mono checkout = /home/su/mono/branches/mono-unity-2018.3-mbe
>> System Info :
Linux ubuntu 3.13.0-32-generic #57-Ubuntu SMP Tue Jul 15 03:51:12 UTC 2014 i686 i686 i686 GNU/Linux
fatal: Not a git repository (or any of the parent directories): .git
fatal: Not a git repository (or any of the parent directories): .git
>>> Mono Revision =
>>> Build Scripts Revision =
>>> Existing Mono =
>>> Mono Arch = i686
>>> Checking out mono build tools extra to : /home/su/mono/branches/mono-unity-2018.3-mbe/../../mono-build-tools-extra/build
>>> Cloning git@github.cds.internal.unity3d.com:unity/mono-build-tools-exta.git at /home/su/mono/branches/mono-unity-2018.3-mbe/../../mono-build-tools-extra/build
Cloning into '/home/su/mono/branches/mono-unity-2018.3-mbe/../../mono-build-tools-extra/build'...
The authenticity of host 'github.cds.internal.unity3d.com (34.76.197.141)' can't be established.
ECDSA key fingerprint is 29:23:e2:73:71:ec:9f:62:76:80:b0:50:48:96:ed:34.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'github.cds.internal.unity3d.com,34.76.197.141' (ECDSA) to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.

Sames likes it need to clone a git repo named mono-build-tools-exta but it's a private repo? Anyone could help? I'm totally a newbie in Linux……