Run build.sh with the environment variable J2OBJC set to your j2objc binary:
J2OBJC=~/j2objc/dist/j2objc ./build.sh
javac compiled successfully
translating outer/Outer.java
error: outer/Outer.java:10: The import java.nio.file cannot be resolved
Translated 0 files: 1 errors, 0 warnings
Translated 0 methods as functions
j2objc compilation failed