builder prefix with "=" doesn't work

[vsoch]

thanks for the catch tin!

**^ tin mintbook128 /opt ^**>  sudo ./singularity-builder/singularity_build.sh all --prefix=/opt/singularity-${VER}
Unknown option: --prefix=/opt/singularity-2.4.1
**^ tin mintbook128 /opt ^**>  sudo ./singularity-builder/singularity_build.sh all --prefix /opt/singularity-${VER}
Cloning into 'singularity'...
remote: Counting objects: 28801, done.
remote: Compressing objects: 100% (198/198), done.
remote: Total 28801 (delta 167), reused 218 (delta 107), pack-reused 28496
Receiving objects: 100% (28801/28801), 5.17 MiB | 2.14 MiB/s, done.
Resolving deltas: 100% (21170/21170), done.
Checking connectivity... done.
Cloned master branch of singularity to /tmp