update installation procedure for EasyBuild in generated Singularity container recipes

[boegel]

fixes #3121

Tested with a minimal container recipe generated via:

eb -C bzip2-1.0.6.eb --experimental --container-config bootstrap=yum,osversion=7

which builds as expected:

$ singularity build --fakeroot bzip2.sif  Singularity.bzip2-1.0.6
...
+ pip install -U pip
...
+ pip install easybuild
...
Successfully installed easybuild-4.3.1 easybuild-easyblocks-4.3.1 easybuild-easyconfigs-4.3.1 easybuild-framework-4.3.1
...
+ eb bzip2-1.0.6.eb --robot
== temporary log file in case of crash /scratch/tmp/eb-CqU3EY/easybuild-J7I25L.log
== resolving dependencies ...
== processing EasyBuild easyconfig /usr/easybuild/easyconfigs/b/bzip2/bzip2-1.0.6.eb
== building and installing bzip2/1.0.6...
...
== COMPLETED: Installation ended successfully (took 5 sec)
== Results of the build can be found in the log file(s) /app/software/bzip2/1.0.6/easybuild/easybuild-bzip2-1.0.6-20201130.142213.log
== Build succeeded for 1 out of 1
...
INFO:    Creating SIF file...
INFO:    Build complete: bzip2.sif

[bartoldeman]

lgtm