specify some channels to solve the conda env faster

[maxulysse]

conda env solving was too slow.

Solving environment: ...working...
Warning: 2 possible package resolutions (only showing differing packages):
- conda-forge/linux-64::llvm-openmp-8.0.1-hc9558a2_0, conda-forge/linux-64::openmp-8.0.1-0
- conda-forge/linux-64::openmp-7.0.0-h2d50403_0, conda-forge/noarch::llvm-meta-7.0.0-0

Asking for the first resolution from the start seems to speed up the process a bit.

nf-core/sarek pull request

Many thanks for contributing to nf-core/sarek!

Please fill in the appropriate checklist below (delete whatever is not relevant).
These are the most common things requested on pull requests (PRs).

PR checklist

• This comment contains a description of changes (with reason)
• If you've fixed a bug or added code that should be tested, add tests!
• If necessary, also make a PR on the nf-core/sarek branch on the nf-core/test-datasets repo
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
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• CHANGELOG.md is updated
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