[AIRFLOW-6171] Apply .airflowignore to correct subdirectories

[ghost]

Fix the defect that applied .airflowignore rules from one subdirectory
to all other subdirectories scanned later.

Make sure you have checked all steps below.

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Tests

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[ashb]

This doesn't actually need a dag for the test to work, so this file can be mostly empty.

[ghost]

It can be almost empty but this is under the dags folder and we're making this file look like a dag to Airflow so it seems to make sense to place an actual test dag here.

[ashb]

Yes and no - we're only testing the ignore behaviour, which we test "in isolation" outside of the rest of the scheduler. Anyway, let's put schedule_interval=None then please so that this doesn't actually run?

[ashb]

Mostly looks good, small change requested though please.

[codecov-io]

Codecov Report

Merging #6784 into master will decrease coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6784      +/-   ##
==========================================
- Coverage   84.53%   84.29%   -0.24%     
==========================================
  Files         672      672              
  Lines       38153    38219      +66     
==========================================
- Hits        32252    32218      -34     
- Misses       5901     6001     +100

Impacted Files	Coverage Δ
airflow/utils/file.py	91.46% <100%> (ø)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	78.2% <0%> (-20.52%)	⬇️
airflow/contrib/operators/ssh_operator.py	83.33% <0%> (-1.29%)	⬇️
airflow/executors/kubernetes_executor.py	57.29% <0%> (-0.64%)	⬇️
airflow/gcp/hooks/bigquery.py	70.71% <0%> (-0.14%)	⬇️
...flow/contrib/operators/gcp_cloud_build_operator.py	100% <0%> (ø)	⬆️
... and 5 more

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