Isolate Aus Cop Hub query as its own function

[caitlinadams]

• Create a new function, query_scene_from_aus_cop_hub to run the query (which does not require credentials)
• Update .env file and aus cop hub functions to look for two new environment variables, which allows us to support running with either conda through Docker or micromamba on NCI:
  • PYGSSEARCH_ENV_EXECUTABLE - the path or alias for the executable, e.g. conda, /path/to/micromamba/bin/micromamba
  • PYGSSEARCH_ENV_NAME - the name of the environment where pygssearch is installed, pygssearch-env by default based on the name in the conda environment yaml
• Update test_scenes.py to add a test for aus cop hub query

Regarding the test I added, there's two things to note:

• The environment executable and environment name are required to run the test. I'm not sure how best to handle this to allow the test to run on GitHub, AWS, and NCI
• One scene fails the aus cop hub query test -- it's an IW scene over Australia that aus cop hub doesn't have.
• I also needed to copy over the logic for getting the environment variables from the download tests to the scene tests because they're now used in the aus cop hub query function. I think we could perhaps generalise this logic into a utility function to allow us to set required variables, load them via dotenv if needed, and report any missing. There's a few places where we could tidy up the code by using a function version of that code.