Some JVMs pass through `home` as a derivation rather than as a string, as `openjdk` does. Since the module option for session variables expects a string, this is a type error. I suspect that this incorrect, and have changed the assignment here to coerce the `cfg.package.home` attribute to a string to be safe.
After discussing with @NobbZ, we have decided it is best to mitigate this problem in HM rather than to make potentially breaking changes to Nixpkgs.
Please do mention if you think we ought to propose a change to Nixpkgs instead.
The previous variant used IFD to generate the `JAVA_HOME` variable and relied on internal hooks of the `java` package, this failed for a user cross compiling their configuration.
This PR changes that and uses the `home` attribute, as documented in the very last sentence of the https://nixos.org/manual/nixpkgs/stable/#sec-language-java chapter.
