title: Error when running cwl wrapper from documentation
body: <p>While running the CWL support example from snakemake documentation:
  <a href="https://snakemake.readthedocs.io/en/stable/snakefiles/modularization.html#common-workflow-language-cwl-support" rel="nofollow noreferrer">https://snakemake.readthedocs.io/en/stable/snakefiles/modularization.html#common-workflow-language-cwl-support</a></p>
  <p>I get the following error output,</p>
  <pre><code>(snakemake) pedro@Gen83-ubuntu:~/projects/sandbox/cwl-test$ snakemake -j12 &quot;mapped/foo.sorted.bam&quot;
  Building DAG of jobs...
  Using shell: /bin/bash
  Provided cores: 12
  Rules claiming more threads will be scaled down.
  Job stats:
  job              count    min threads    max threads
  -------------  -------  -------------  -------------
  samtools_sort        1              8              8
  total                1              8              8
  
  Select jobs to execute...
  
  [Wed Oct 20 11:20:03 2021]
  rule samtools_sort:
      input: mapped/foo.unsorted.bam
      output: mapped/foo.sorted.bam
      jobid: 0
      wildcards: sample=foo
      threads: 8
      resources: tmpdir=/tmp
  
  /usr/bin/cwltool 1.0.20180302231433
  I'm sorry, I couldn't load this CWL file, try again with --debug for more information.
  The error was: mapping values are not allowed here
    in &quot;https://github.com/common-workflow-language/workflows/blob/fb406c95/tools/samtools-sort.cwl&quot;, line 137, column 65
  </code></pre>
  <p>The Snakefile is almost a copy-paste of the documentation code:</p>
  <pre><code>rule samtools_sort:
      input:
          input=&quot;mapped/{sample}.unsorted.bam&quot;
      output:
          output_name=&quot;mapped/{sample}.sorted.bam&quot;
      params:
          threads=lambda wildcards, threads: threads,
          memory=&quot;4G&quot;
      threads: 8
      cwl:
          &quot;https://github.com/common-workflow-language/workflows/blob/&quot;
          &quot;fb406c95/tools/samtools-sort.cwl&quot;
  </code></pre>
  <p>Snakemake and cwltool version in the conda envinronment are,</p>
  <pre><code>(snakemake) pedro@Gen83-ubuntu:~/projects/sandbox/cwl-test$ cwltool --version
  /usr/bin/cwltool 1.0.20180302231433
  (snakemake) pedro@Gen83-ubuntu:~/projects/sandbox/cwl-test$ snakemake --version
  6.9.1
  </code></pre>
  <p>How can run this example successfully?</p>