colorbar env variable not captured in cset_workflow

[refszkentla]

Describe the bug

Not a great bug, easy solvable, easy managable by the user within cset_workflow.
COLORBAR_FILE variable is not promoted to flow.cylc env. So everything works fine,
except that user style file is int recognized/used.

Also, there is other bug connected with this one.
Once the COLORBAR_FILE = {{COLORBAR_FILE}} sits in root section of flow.cylc, the style file option is triggered, however, the variable is wrapped with single quotes ' ' and the value not recognized as file path.
So also these two quotes need to be removed from around app/bake_recipes/bin/bake.sh style-file value.

How to reproduce

Steps to reproduce the behaviour:

1. run any CSET workflow with user defined colorbar .json file (make ome changes to colors or ranges in user defined .json before, of course).
2. suite runs, plots ignore user colors/ranges
3. run again CSET by just adding COLORBAR_FILE = {{COLORBAR_FILE}} in root section of flow.cylc before; this triggers the usage of --style-file option but workflow fails, because quoted style-file is treated probably like a path with quotes, e.g. 'user.json`.

Expected behaviour

User defined colorbar to be used and properly recognized.

[jfrost-mo]

Thank you for the very detailed bug report, it made solving this nice and fast. I've sent the fix in #2029 your way for review.