cwltool fail when multiple jobs try to use the same conda env

I tried to run workflow in a cluster using a shared conda installation.
I have created all conda envs required by the workflow.
Full command:

```
cwltool --disable-js-validation --skip-schemas --no-container --beta-conda-dependencies --beta-dependencies-directory /path/to/folder/with/_conda/ --rm-tmpdir --tmpdir-prefix=/eTMP/1559223537/ /download_quality_control_single.cwl sra_download_SRR10043607.yml
```

Running multiple jobs at the same time produce this error:
```
INFO [workflow ] start
INFO [workflow ] starting step fastq_dump
INFO [step fastq_dump] start
INFO [job fastq_dump] /export/TMP/1559223537/6jb6l69q$ fastq-dump \
--gzip \
--split-files \
SRR10043607 \
--defline-seq \
'@$ac.$si/$ri' \
--skip-technical
ERROR Exception while running job: Failed to get file lock for /path/to/folder/with/conda.
WARNING [job fastq_dump] completed permanentFail
ERROR [step fastq_dump] Output is missing expected field file:////download_quality_control_single.cwl#download_quality_control/fastq_dump/output
WARNING [step fastq_dump] completed permanentFail
INFO [workflow ] completed permanentFail
{
"fastq": null,
"fastqc_html": null,
"fastqc_zip": null
}WARNING Final process status is permanentFail
```

How can I make cwltool to use conda but not to lock the conda dir?