pin htslib in ensemblvep/vep (#11770)

* Add htslib dependency to environment.yml

* Apply suggestion from @maxulysse

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Co-authored-by: Maxime U Garcia <max.u.garcia@gmail.com>

Author: matthdsm

modules/nf-core/ensemblvep/vep/environment.yml

@@ -6,3 +6,4 @@ channels:
 dependencies:
   - bioconda::ensembl-vep=115.2
   - bioconda::perl-math-cdf=0.1
+  - bioconda::htslib=1.21