Gather test coverage with stable rust

[ricohageman]

This will ensure that the test coverage can be determined using the stable rust release.

[davidhewitt]

Thanks 👍

[davidhewitt]

Not sure why the coverage jobs are failing... ?

[adamreichold]

Not sure why the coverage jobs are failing... ?

I think our xtask binary is still forcing a nightly toolchain:

pyo3/xtask/src/llvm_cov.rs

Lines 92 to 97 in 749fe6c

	let rustc_version =
	String::from_utf8(get_output(Command::new("rustc").arg("--version"))?.stdout)
	.context("failed to parse rust version as utf8")?;
	if !rustc_version.contains("nightly") {
	env.insert("RUSTUP_TOOLCHAIN".to_owned(), "nightly".to_owned());
	}