support `${HOME}` being ldmx-sw itself

[tomeichlersmith]

This is work towards resolving LDMX-Software/ldmx-sw#1472 by eventually dropping support for ${HOME} in the container being ldmx-sw parent directory.

In order to avoid breaking current workflows, I am going to start by supporting both with some additional scripting within the ldmx-env-init.sh script.

[tomeichlersmith]

Checks

• HOME is ldmx-sw parent (i.e. ldmx-sw trunk)
  • configure, done by CI in this repo
  • build, done by CI in this repo
  • basic simulation run, done by CI in this repo
  • test
  • GENIE run
• HOME is ldmx-sw (i.e. ldmx-sw 1472)
  • configure
  • build
  • basic simulation run
  • test
  • GENIE run
• custom Geant4: check that the deduced env is the same transition custom Geant4 to be documentation based, have users edit their local copy of .profile

Notes

• Transitioning from parent-directory-is-denv to ldmx-sw-is-denv needs to be done proactively. I want to avoid folks having a denv in both locations. Looking at writing a recipe for this.