test-distro: build without cross toolchain

Make the xz2 dependency optional to allow building without a C cross
compiler. This allows clippy.sh to be used on e.g. macOS more easily:

```
./clippy.sh --target x86_64-unknown-linux-gnu --exclude-features xz2
```

Author: tamird

test-distro/Cargo.toml

@@ -35,4 +35,4 @@ nix = { workspace = true, features = [
 ] }
 object = { workspace = true, features = ["elf", "read_core", "std"] }
 walkdir = { workspace = true }
-xz2 = { workspace = true }
+xz2 = { workspace = true, optional = true } # Optional to allow building without a cross toolchain.

test-distro/src/lib.rs

@@ -37,17 +37,25 @@ pub fn read_to_end(path: &std::path::Path, compressed: bool) -> anyhow::Result<V
     let mut contents = Vec::new();
 
     if compressed {
-        let stat = f.metadata().context("metadata()")?;
-        #[expect(clippy::manual_ok_err)]
-        let len = match usize::try_from(stat.len()) {
-            Ok(len) => Some(len),
-            Err(std::num::TryFromIntError { .. }) => None,
+        #[cfg(feature = "xz2")]
+        {
+            let stat = f.metadata().context("metadata()")?;
+            #[expect(clippy::manual_ok_err)]
+            let len = match usize::try_from(stat.len()) {
+                Ok(len) => Some(len),
+                Err(std::num::TryFromIntError { .. }) => None,
+            }
+            .and_then(|len| len.checked_mul(2))
+            .ok_or_else(|| anyhow::anyhow!("2 * {stat:?}.len() is too large to fit in a usize"))?;
+            contents.reserve(len);
+
+            xz2::read::XzDecoder::new(f).read_to_end(&mut contents)
         }
-        .and_then(|len| len.checked_mul(2))
-        .ok_or_else(|| anyhow::anyhow!("2 * {stat:?}.len() is too large to fit in a usize"))?;
-        contents.reserve(len);
 
-        xz2::read::XzDecoder::new(f).read_to_end(&mut contents)
+        #[cfg(not(feature = "xz2"))]
+        {
+            anyhow::bail!("cannot read {} without xz2 feature", path.display());
+        }
     } else {
         f.read_to_end(&mut contents)
     }