Add workaround for Snakemake bug #437

Job	Run time
quick-tests (sanity)	1m 32s
quick-tests (dry-run)	1m 13s
quick-tests (utilities)	1m 25s
quick-tests (report)	32s
quick-tests (scripts)	35s
quick-tests (docs)	40s
functional-tests (qc-seq)	22m 23s
functional-tests (snv-indels)	8m 54s
functional-tests (itd)	6m 38s
functional-tests (fusion)	6m 51s
functional-tests (expression)	6m 47s
functional-tests (test-hamlet-targetted-RNA)	7m 4s
functional-tests (test-hamlet-chrM)	4m 39s
	1h 9m 13s

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