Fix vars and input that references app context

[m4dcoder]

Fix the construction sequence of the workflow initial context. Assign the app context as the initial context before the workflow input and vars are rendered. This allows expressions in vars and input to reference variables set in the app context.

[codecov-io]

Codecov Report

Merging #104 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
+ Coverage   94.24%   94.24%   +<.01%     
==========================================
  Files          38       38              
  Lines        2379     2381       +2     
  Branches      470      470              
==========================================
+ Hits         2242     2244       +2     
  Misses         79       79              
  Partials       58       58

Impacted Files	Coverage Δ
orquesta/specs/native/v1/models.py	97.63% <100%> (ø)	⬆️
orquesta/conducting.py	91.98% <100%> (+0.03%)	⬆️
orquesta/specs/mistral/v2/workflows.py	83.01% <100%> (ø)	⬆️

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[blag]

LGTM