SignedCapsulePkg: Use BaseCryptLibNull to reduce package CI time

SignedCapsulePkg: Use BaseCryptLibNull to reduce package CI time

https://bugzilla.tianocore.org/show_bug.cgi?id=2315

Use BaseCryptLibNull for package CI builds to reduce package
build times.  Enabled with PYTOOL_CONTINUOUS_INTEGRATION in YAML
files.  By default PYTOOL_CONTINUOUS_INTEGRATION is not defined,
and the original lib mappings are preserved.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>