enos: make the local build target configurable (#2313)

This makes the `make` build target that is used for local builder
variant configurable. This will allow us to utilize the local builder
variant in downstreams that might use different targets for their build.

* Make the local build target configurable
* Allow cross compiling `linux/amd64` on different platforms.

scripts/build.sh

@@ -88,4 +88,4 @@ fi
 
 # Done!
 echo "==> Results:"
-ls -hl ${BIN_PARENT_DIR}
+ls -hl ${BIN_PARENT_DIR}