How can I point Nextflow to the correct singularity binary?

I have a Nextflow workflow which I run from a conda environment (which also has singularity installed):

name: nextflow
channels:
  - conda-forge
  - bioconda
  - defaults
dependencies:
  - nextflow
  - singularity

The relevant snippet on nextflow.config is:

profiles {
    local {
        executor {
            name = "local"
        }
        includeConfig 'configs/local.config'
    }

    singularity {
        // Included since conda is still needed for some processes
        conda.enabled = true
        timeout = '2 h'
        // singularity-specific configs
        singularity.enabled = true
        singularity.autoMounts = true
        includeConfig 'configs/singularity.config'
    }
}

Before running my workflow, I activate the nextflow Conda environment. Still, when I run the workflow, I get:

ERROR ~ Error executing process > 'preprocessing:fastpTrimming (5)'

Caused by:
  Process `cleaning_stuff` terminated with an error exit status (127)

Command executed:

  command --option --another_option -out /some/path

Command exit status:
  127

Command output:
  (empty)

Command error:
  env: 'singularity': No such file or directory

Work dir:
  /path/to/work/dir

Tip: view the complete command output by changing to the process work dir and entering the command `cat .command.out`

 -- Check '.nextflow.log' file for details

It seems Nextflow doesn't “see” singularity installed via Conda.
How can I point Nextflow to the correct singularity binary?

I don't think it is possible to mix using Singularity containers and Conda environments within a single nextflow execution. I think using conda.enabled = true inside your singularity config profile might be problematic here. If you're using the local executor and don't yet specify a container directive for all processes, simply install the required tools/packages (for which you don't yet have containers for) into your local nextflow conda environment.

Note that the Bioconda build system also builds Docker containers and these are uploaded to quay.io as part of the Biocontainers project. Since Singularity/Apptainer can pull Docker containers, you may like to use one or more of these containers in your workflow. For example, in your nextflow.config`:

process {

    withName: 'fastpTrimming' {

        container = 'quay.io/biocontainers/fastp:0.23.4--hadf994f_2'
    }

    ...
}

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