Ensure correct output formats of seqtk_seq and seqtk_mergefa in all cases #15289

Job	Run time
Setup cache and determine changed repositories (3.11)	1m 40s
Lint Python scripts (3.11)	30s
Lint tool-list (3.11)	58s
Lint R scripts (ubuntu-20.04, release)	1m 57s
Check file sizes	42s
Test tools (0, 3.11)	4m 4s
Test tools (1, 3.11)	5m 25s
Test tools (2, 3.11)	5m 32s
Test tools (3, 3.11)	4m 9s
Combine chunked test results (3.11)	34s
Check workflow success	1s
Deploy	0s
Report deploy status	0s
	25m 32s

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