Use the correct build JDK details when writing the Spring Boot MANIFEST.MF file

[plaird]

See write_manifest.sh. It helpfully inscribes the version of the JDK that was used to build the code in the generated Spring Boot application. Unfortunately, the value that is being written today is incorrect in many cases.

https://github.com/salesforce/bazel-springboot-rule/blob/master/tools/springboot/write_manifest.sh#L28

We need to inject this information from the actual JDK used to build the source (the toolchain), not the JDK that is in path while the genrule executes.

[plaird]

This will be easy to fix once Java toolchains are implemented correctly in Bazel. They just finished implementation of it in master yesterday.

Java Toolchain resolution in Bazel

We will have to figure out how to use that feature with backwards compatibility in mind. Not all users will be on the latest Bazel.