pip-requirements.txt: Update edk2-pytool-library to 0.12.0

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4146



Update edk2-pytool-library to version 0.12.0 that adds support for
the environment variable PYTOOL_TEMPORARILY_IGNORE_NESTED_EDK_PACKAGES
that can be set to true to ignore nested packages instead of breaking
the build with an exception. Nested packages are not allowed by the
edk2 specifications.  This environment variable allows pytools to run
with reduced functionality if nested packages are present giving
downstream consumers of edk2 that use pytools time to resolve the use
of nested packages and restore all features of pytools.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>