nf-core download has wrong name for singularity converted containers

### Description of the bug

nf-core download sarek produces
nf-core-gatk-4.4.0.0.img

nextflow run nf-core/sarek produces
quay.io-nf-core-gatk-4.4.0.0.img

### Command used and terminal output

_No response_

### System information

_No response_