Update Package.swift to point to temporary zips for testing

Package.swift

@@ -24,10 +24,11 @@ let package = Package(
   targets: [
     .binaryTarget(
       name: "abseil",
-      // TODO(ncooke3): Update the `url` with the actual value.
-      url: "URL_TO_BINARY",
-      // TODO(ncooke3): Update the `checksum` with the actual value.
-      checksum: "CHECKSUM"
+      // TODO(ncooke3): Temporarily hosting on GitHub while wiring everything
+      // up. Set up hosting elsewhere and point to resulting URL.
+      url: "https://github.com/google/abseil-cpp-binary/releases/download/10.8.0-rc.0/abseil.zip",
+      // TODO(ncooke3): Update the `checksum` when updating above URL.
+      checksum: "bba6a353ff3f6f70264b507b05a2d65de09f624da80d437b76ff37115bfc0bba"
     )
   ]
 )