yaml variable needed for preanalysis scripts

[ceblanton]

From @nikizadehgfdl -

Using the 2026.01.alpha2 pre-release, @nikizadehgfdl found that the gfdlvitals preAnalysis script works, but cannot find the desired output directory.

The desired output directory is:

/nbhome/$USER/FRE_STEM/EXPNAME/platform-target

The user scripts will have to parse the yaml to read the user setting, in this case, the nbhome_root variable.

[ceblanton]

After discussion, we decided to support this through two related mechanisms, and not pass through this nbhome_root variable.

1. Reference the serialized model yaml that is in the installed cylc directory, say by setting an environment variable yaml that user scripts can read
2. Allow user-specified variables in the model yaml to persist beyond serialization (i.e. not cleaning them during the yaml combining/serialization)

[ceblanton]

I'm going to change the name and content of this issue for (1) above, and the fre-cli issue for (2) is NOAA-GFDL/fre-cli#851

[ilaflott]

oops my bad