GitHub Actions fail when attempting to locate external files

[stratofax]

The ballot generation process requires that the program access external files, like PNG files. Although this process works on my Mac and Linux machines, these tests fail when running in GitHub Actions.

Error output:

Run poetry run pytest
============================= test session starts ==============================
platform linux -- Python 3.9.13, pytest-7.1.2, pluggy-1.0.0
rootdir: /home/runner/work/BallotLabFork/BallotLabFork
plugins: cov-3.0.0
collected 12 items

tests/test_cli.py ......                                                 [ 50%]
tests/test_demo.py F                                                     [ 58%]
tests/test_election_data.py ...                                          [ 83%]
tests/test_make_ballots.py .                                             [ 91%]
tests/test_validate_edf.py .                                             [100%]

=================================== FAILURES ===================================
__________________________________ test_demo ___________________________________

    def test_demo():
>       assert make_demo_ballot() == NO_ERRORS

tests/test_demo.py:6: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
src/electos/ballotmaker/demo.py:11: in make_demo_ballot
    build_ballot()
src/electos/ballotmaker/ballots/demo_ballot.py:126: in build_ballot
    inst = Instructions()
src/electos/ballotmaker/ballots/instructions.py:182: in __init__
    build_instruction_list()
src/electos/ballotmaker/ballots/instructions.py:63: in build_instruction_list
    image1 = EmbeddedImage("filled_bubble.png", image_width)
src/electos/ballotmaker/ballots/images.py:25: in __init__
    self.file_check(image_file)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <electos.ballotmaker.ballots.images.EmbeddedImage object at 0x7f44afb06610>
image_file = <electos.ballotmaker.ballots.files.FileTools object at 0x7f44afb06b20>

    def file_check(self, image_file):
        if image_file.file_found is False:
            file_error = "File {} not found in {}".format(
                self.image_name, self.rel_img_path
            )
>           raise FileNotFoundError(file_error)
E           FileNotFoundError: File filled_bubble.png not found in assets/img

src/electos/ballotmaker/ballots/images.py:50: FileNotFoundError
=========================== short test summary info ============================
FAILED tests/test_demo.py::test_demo - FileNotFoundError: File filled_bubble....
========================= 1 failed, 11 passed in 0.44s =========================
Error: Process completed with exit code 1.