Refactor: Allow modifiable download dest path

This adds the parameter `archive_dest` to `run_installer`, which will
control where `helper.downloadBinaryFile` will download files, and sets
it to "." by default.

For convenience/readability, this will also turn any reference to the
downloaded file into a `pathlib.Path` object. This makes it easier to
ensure that the file ends up in the right location.

tests/test_install.py

@@ -146,11 +146,11 @@ def make_mock_geturl_download_archive(
 
         def locate_archive() -> MockArchive:
             for arc in archives:
-                if out == arc.filename_7z:
+                if Path(out).name == arc.filename_7z:
                     return arc
             assert False, "Requested an archive that was not mocked"
 
-        locate_archive().write_compressed_archive(Path("./"))
+        locate_archive().write_compressed_archive(Path(out).parent)
 
     return mock_getUrl, mock_download_archive
 
@@ -731,6 +731,7 @@ def test_install_installer_archive_extraction_err(monkeypatch):
             base_dir=temp_dir,
             command="some_nonexistent_7z_extractor",
             queue=MockMultiprocessingManager.Queue(),
+            archive_dest=Path(temp_dir),
         )
     assert err.type == ArchiveExtractionError
     err_msg = format(err.value).rstrip()