Improve how variables are passed through to avoid quoting issues

### What problem does your feature request solve?

Currently a lot of issues arise from various shells interpreting the variables to be put into the templates. This is confusing, and hard to get right, so should be improved.

### Describe the solution you'd like

Ideally nothing between rose edit and CSET should be parsing these values, so we should probably encode them somehow and then send them across directly. Maybe JSON encoding, or something else?

### Describe alternatives you've considered

Much pain.

Start by tracing how variables move between rose edit and CSET, then reproduce the shell-quoting failures described in the issue. Done means variable values cross that boundary without unintended shell parsing; the issue does not name specific files or tests.

Written by the indexing model from the issue text.