Configure spack install dir based on env_name

[linsword13]

Configure spack install dir based on env_name

This helps with saving redundant spack concretization, when experiments
under different workloads are actually using the same software environment.

Some more contexts (courtesy of Bob and Doug):

• Historically the spack env was rendered during workspace render time,
  that's why the existing namespacing.

• That's changed in Convert environment generation logic to lazy / implcit instead of explicit #433.

• And it's safe to assume same env_name always 1-1 maps to a spack
  env. There's a check to guarantee that:

  ramble/lib/ramble/ramble/software_environments.py

  Lines 371 to 374 in f4e0e06

  	raise RambleSoftwareEnvironmentError(
  	f'Environment {name} defined multiple times in inconsistent '
  	f'ways.\nPackage with differences is {rendered_pkg.name}'
  	)

  .

Tested: tested with the following:

• A new created workspace with gromacs ran fine.

• (backward-compt) An existing workspace created from the current
  develop head ran fine after running ramble workspace setup under the
  current branch.