Make Nomulus compile on macOS (#1421)

BSD sed requires a parameter to -i to indicate the backup suffix. By
adding a blank suffix the sed command works on both Linux and macOS.

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core/build.gradle

@@ -470,7 +470,7 @@ task soyToJava {
 
     outputs.each { file ->
       exec {
-        commandLine 'sed', '-i', 's/@link/LINK/g', file.getCanonicalPath()
+        commandLine 'sed', '-i""', '-e', 's/@link/LINK/g', file.getCanonicalPath()
       }
     }
   }