support spack 1.2 to stackinator v6

[bcumming]

Add support for spack 1.2 in stackinator 6

This adds support for quirks of Spack 1.2 to the current version of stackinator that can targets Spack 1.0 and 1.1.

• disable the new Spack installer (making it work properly requires deep plumbing changes that might affect compatibility with 1.0 and 1.1.
• insert the --yes-to-all flag to uenv buildcache calls when using Spack 1.2

This also improves how the Spack version is determined and used in the Jinja templates.

Note that this is separate from Stackinator v7, which supports Spack 1.2 more thoroughly (with no backwards compatibility with v1.1 and v1.0): #298