define $CONDA_PREFIX via modextravars in QIIME2

[lexming]

(created using eb --new-pr)

edit: $CONDA_PREFIX is not a path-like variable, so setting it via modextrapaths is incorrect.
Moreover, the changes in easybuilders/easybuild-framework#4868 broke the QIIME2 easyconfigs, because the unifrac extension requires that $CONDA_PREFIX is set to something, and the new module_load_environment approach doesn't set $CONDA_PREFIX if the installation directory is just an empty directory when it's set via modextrapaths (see also discussion in #22905)

[lexming]

@boegelbot please test @ jsc-zen3

[boegelbot]

@lexming: Request for testing this PR well received on jsczen3l1.int.jsc-zen3.fz-juelich.de

PR test command 'if [[ develop != 'develop' ]]; then EB_BRANCH=develop ./easybuild_develop.sh 2> /dev/null 1>&2; EB_PREFIX=/home/boegelbot/easybuild/develop source init_env_easybuild_develop.sh; fi; EB_PR=22918 EB_ARGS= EB_CONTAINER= EB_REPO=easybuild-easyconfigs EB_BRANCH=develop /opt/software/slurm/bin/sbatch --job-name test_PR_22918 --ntasks=8 ~/boegelbot/eb_from_pr_upload_jsc-zen3.sh' executed!

• exit code: 0
• output:

Submitted batch job 6400

Test results coming soon (I hope)...

- notification for comment with ID 2894531370 processed

Message to humans: this is just bookkeeping information for me,
it is of no use to you (unless you think I have a bug, which I don't).

[boegel]

lgtm

[boegelbot]

Test report by @boegelbot
SUCCESS
Build succeeded for 4 out of 4 (3 easyconfigs in total)
jsczen3c1.int.jsc-zen3.fz-juelich.de - Linux Rocky Linux 9.5, x86_64, AMD EPYC-Milan Processor (zen3), Python 3.9.21
See https://gist.github.com/boegelbot/8fdbd35b479b71c4f86cf7d2945a0944 for a full test report.