.pytool/EccCheck: Set PACKAGES_PATH env var in Ecc

.pytool/EccCheck: Set PACKAGES_PATH env var in Ecc

When running Ecc on other repositories (e.g.: edk2-platforms with
edk2 as a submodule), edk2 modules are referenced.
E.g.: MdePkg/..
The PACKAGES_PATH env var can be used to reference other directories
containing packages. Set it so that Ecc can find these packages.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>