More robust method for getting cyclus installation directory

[bennibbelink]

Closes #1688.

Instead of relying on the environment variable _ to get the path of the running application (which returns the python installation directory when cyclus is called via python) this calls which cyclus to find the location of the cyclus binary.

This assumes that the machine has the which utility and that the cyclus installation directory is in the PATH. Not 100% robust but I think these are assumptions we can safely make (or enforce on our users).

[github-actions]

Downstream Build Status Report

Build FROM cyclus_20.04_apt/cyclus

• Cycamore: Success ✅
• Cymetric: Success ✅

Build FROM cyclus_20.04_conda/cyclus

• Cycamore: Success ✅
• Cymetric: Success ✅

Build FROM cyclus_22.04_apt/cyclus

• Cycamore: Success ✅
• Cymetric: Success ✅

Build FROM cyclus_22.04_conda/cyclus

• Cycamore: Success ✅
• Cymetric: Success ✅

[coveralls]

Pull Request Test Coverage Report for Build 8218293399

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 15 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.001%) to 32.589%

---

Files with Coverage Reduction	New Missed Lines	%
src/env.cc	15	36.56%

Totals
Change from base Build 8194137311:	0.001%
Covered Lines:	52974
Relevant Lines:	128976

---

💛 - Coveralls

[gonuke]

Seems like there must be a better way 😁 but this looks ok to me.