let valet use only one thread for samtools sort

[bernt-matthias]

with -@ 16 I get samtools sort: couldn't allocate memory for bam_mem

seem better anyway with restrictive HPC setups

reported upstream marbl/VALET#12

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[bernt-matthias]

@BiocondaBot please fetch artifacts

[BiocondaBot]

Package(s) built on CircleCI are ready for inspection:

Arch	Package	Repodata
noarch	valet-1.0-3.tar.bz2	repodata.json

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Docker image(s) built:

Package	Tag	Install with docker
valet	1.0--3	show curl "https://117902-42372094-gh.circle-artifacts.com/0/tmp/artifacts/images/valet%3A1.0--3.tar.gz" | gzip -dc | docker load

[bernt-matthias]

@BiocondaBot please add label

[bgruening]

@bioconda-bot please merge

[BiocondaBot]

I will attempt to upload artifacts and merge this PR. This may take some time, please have patience.