skawarePackages.buildPackage: make sha256 optional

Fall back to fakeSha256 if unspecified, like many other fetcher
functions now do, to streamline packages / new updates.

Changed files
+1 -1
pkgs
development
skaware-packages
+1 -1
pkgs/development/skaware-packages/build-skaware-package.nix
···
# : string
, version
# : string
-
, sha256
+
, sha256 ? lib.fakeSha256
# : string
, description
# : list Platform