Create ~/.rpmmacros file on install for Ubuntu operating systems

This temporary (?) fix [1] allows yum-based bootstrapping of Singularity
containers on Ubuntu (and/or Debian-based) operating systems.

[1]

ubuntu@singualrity-ubuntu-builder:~/naked-singularity/definition-files/centos$ sudo singularity build centos-7.9.2009.sif Singularity.centos-7.9.2009
INFO:    Starting build...
FATAL:   While performing build: conveyor failed to get: while checking rpm path: RPM database is using a weird path: %(bash
You are probably running this bootstrap on Debian or Ubuntu.
There is a way to work around this problem:
Create a file at path /home/ubuntu/.rpmmacros.
Place the following lines into the '.rpmmacros' file:
%_var /var
%_dbpath %{_var}/lib/rpm
After creating the file, re-run the bootstrap.
More info: https://github.com/hpcng/singularity/issues/241