UnitTestFrameworkPkg: Use TianoCore mirror of cmocka repository

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 Barkelew <Bret.Barkelew@microsoft.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Andrew Fish <afish@apple.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>