REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3301

The cmocka repository https://git.cryptomilk.org/projects/cmocka.git
has gone down a few times in past year.  When it is down, it blocks
EDK II CI.  A mirror of this repository has been created in the
TianoCore organization at https://github.com/tianocore/edk2-cmocka.git
and uses a GitHub Action to auto-sync changes from
https://git.cryptomilk.org/projects/cmocka.git.

* Update .gitmodules to use https://github.com/tianocore/edk2-cmocka.git
  instead of https://git.cryptomilk.org/projects/cmocka.git.

* Update README.rst to reference the COPYING file in
  https://github.com/tianocore/edk2-cmocka.git with the cmocka license.

* Update Azure Pipelines YML files to remove a temporary workaround that
  used an alternate GitHub mirror of cmocka.  With the workaround removed,
  EDK II CI always uses the TianoCore mirror of cmocka.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkele...