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
```

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

@@ -37,6 +37,8 @@ pub fn read_to_end(path: &std::path::Path, compressed: bool) -> anyhow::Result<V
     let mut contents = Vec::new();
 
     if compressed {
+        #[cfg(feature = "xz2")]
+        {
             let stat = f.metadata().context("metadata()")?;
             #[expect(clippy::manual_ok_err)]
             let len = match usize::try_from(stat.len()) {
@@ -48,6 +50,12 @@ pub fn read_to_end(path: &std::path::Path, compressed: bool) -> anyhow::Result<V
             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)
     }

xtask/src/run.rs

@@ -338,9 +338,10 @@ pub fn run(opts: Options) -> Result<()> {
 
                 let target = format!("{guest_arch}-unknown-linux-musl");
 
-                let test_distro: Vec<(String, PathBuf)> = build(Some(&target), |cmd| {
-                    cmd.args(["--package", "test-distro", "--profile", "release"])
-                })
+                let test_distro_args =
+                    ["--package", "test-distro", "--release", "--features", "xz2"];
+                let test_distro: Vec<(String, PathBuf)> =
+                    build(Some(&target), |cmd| cmd.args(test_distro_args))
                         .context("building test-distro package failed")?;
 
                 let binaries = binaries(Some(&target))?;
@@ -415,16 +416,9 @@ pub fn run(opts: Options) -> Result<()> {
                 // Preparing the `modules.alias` file inside the VM as part of
                 // `/init` is slow. It's faster to prepare it here.
                 Command::new("cargo")
-                    .args([
-                        "run",
-                        "--package",
-                        "test-distro",
-                        "--bin",
-                        "depmod",
-                        "--release",
-                        "--",
-                        "-b",
-                    ])
+                    .arg("run")
+                    .args(test_distro_args)
+                    .args(["--bin", "depmod", "--", "-b"])
                     .arg(&modules_dir)
                     .status()
                     .context("failed to run depmod")?;