Spack-builder: `singularity run spack-builder:ubuntu20.10 spack install qt` should work

[wdconinc]

Steps to reproduce the issue

singularity run -B /cvmfs:/cvmfs docker://electronioncollider/spack-builder:ubuntu20.10 spack install qt

This currently returns

==> Error: Failed to acquire a write lock for pkgconf due to LockROFileError: Can't take write lock on read-only file: /cvmfs/eic.opensciencegrid.org/packages/.spack-db/prefix_lock
==> Error: Can't take write lock on read-only file: /cvmfs/eic.opensciencegrid.org/packages/.spack-db/prefix_lock

It should do the equivalent of

singularity run -B /cvmfs:/cvmfs docker://electronioncollider/spack-builder:ubuntu20.10 /bin/bash
source /cvmfs/eic.opensciencegrid.org/packages/spack/current/share/spack/         
spack install qt

Information on your system

$ singularity --version
2.6.1-dist

Additional information

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have run spack maintainers <name-of-the-package> and @mentioned any maintainers
• I have uploaded the build log and environment files
• I have searched the issues of this repo and believe this is not a duplicate