ops: change 'shasum -a 256' to cross-platform 'sha256sum'

ops/build.sh

@@ -13,9 +13,10 @@ STAMP="bin/.golint.stamp"
 # is excluded -- fixtures are not compiled into the binary.
 current_stamp() {
   {
-    find . -type f ! -path '*/testdata/*' | sort | xargs shasum -a 256
-    shasum -a 256 .custom-gcl.yaml
-  } | shasum -a 256 | cut -d' ' -f1
+    find pkg/ -type f ! -path '*/testdata/*' | sort | xargs sha256sum
+    sha256sum .custom-gcl.yaml
+    sha256sum root.go
+  } | sha256sum | cut -d' ' -f1
 }
 
 if [ "$1" = "--if-needed" ] &&