fix lint error for v1 recipes with conditionals in run_exports

[minrk]

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prior to this, recipe-lint failed on a recipe with:

  File "/Users/minrk/dev/conda-forge/conda-smithy/conda_smithy/lint_recipe.py", line 749, in main
    results, hints = lintify_meta_yaml(
                     ^^^^^^^^^^^^^^^^^^
  File "/Users/minrk/dev/conda-forge/conda-smithy/conda_smithy/lint_recipe.py", line 343, in lintify_meta_yaml
    lint_pin_subpackages(
  File "/Users/minrk/dev/conda-forge/conda-smithy/conda_smithy/linter/lints.py", line 661, in lint_pin_subpackages
    check_pins_build_and_requirements(meta)
  File "/Users/minrk/dev/conda-forge/conda-smithy/conda_smithy/linter/lints.py", line 659, in check_pins_build_and_requirements
    check_pins(run_export_section)
  File "/Users/minrk/dev/conda-forge/conda-smithy/conda_smithy/linter/lints.py", line 610, in check_pins
    for pin in (
  File "/Users/minrk/dev/conda-forge/conda-smithy/conda_smithy/linter/lints.py", line 611, in <genexpr>
    pin for pin in all_pins if pin.startswith(filter_pin)
                               ^^^^^^^^^^^^^^
AttributeError: 'CommentedMap' object has no attribute 'startswith'

Same as #2197, but adds a case not yet covered by that one. Test covered by adding recipe from conda-forge/fenics-dolfinx-feedstock#99