Respect PARQUET_TEST_DATA in variant_interop test

[alamb]

Which issue does this PR close?

We generally require a GitHub issue to be filed for all bug fixes and enhancements and this helps us generate change logs for our releases. You can link an issue to this PR using the GitHub syntax.

• closes Error verifying parquet-variant crate on 55.2.0 with verify-release-candidate.sh #7746

Rationale for this change

The parquet-variant tests fail when run as part of verify-release-candidate.sh due to the parquet-testing directory being checked out in a different location

What changes are included in this PR?

Update the test to look at the PARQUET_TEST_DATA environment variable as well

How are these changes tested?

I tested this manually:

# note this is a different name than the submodule:
git clone https://github.com/apache/parquet-testing.git parquet-testing-data
export PARQUET_TEST_DATA=$PWD/parquet-testing-data/data
# checkout my fork
git clone https://github.com/alamb/arrow-rs.git
cd arrow-rs
# This fails on main
git checkout main
cargo test -p parquet-variant
# PASSES on branch with fix
git checkout alamb/fix_variant_tests
cargo test -p parquet-variant

Are there any user-facing changes?

No this is a test only change

[alamb]

This is largely copied from test_util.rs in arrow

arrow-rs/arrow/src/util/test_util.rs

Lines 85 to 105 in 6d01dd9

	/// Returns the parquest test data directory, which is by default
	/// stored in a git submodule rooted at
	/// `arrow/parquest-testing/data`.
	///
	/// The default can be overridden by the optional environment variable
	/// `PARQUET_TEST_DATA`
	///
	/// panics when the directory can not be found.
	///
	/// Example:
	/// ```
	/// let testdata = arrow::util::test_util::parquet_test_data();
	/// let filename = format!("{}/binary.parquet", testdata);
	/// assert!(std::path::PathBuf::from(filename).exists());
	/// ```
	pub fn parquet_test_data() -> String {
	match get_data_dir("PARQUET_TEST_DATA", "../parquet-testing/data") {
	Ok(pb) => pb.display().to_string(),
	Err(err) => panic!("failed to get parquet data dir: {err}"),
	}
	}

However I didn't want to add a dependency on arrow for this crate (even for development)

[alamb]

Thank you for the review @crepererum