fix(juicertools/pre): correctly export java memory options

[prototaxites]

Correctly export the _JAVA_OPTIONS envvar so that juicertools can allocate memory.

PR checklist

Closes #XXX

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
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    • nf-core modules test <MODULE> --profile docker
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    • nf-core subworkflows test <SUBWORKFLOW> --profile docker
    • nf-core subworkflows test <SUBWORKFLOW> --profile singularity
    • nf-core subworkflows test <SUBWORKFLOW> --profile conda

[prototaxites]

Thanks @mashehu!