Work in terms of bytes when patching shebang lines

As [Uv's implementation](https://github.com/astral-sh/uv/blob/db14cc3005d2cd53802cb04c2f1e177a22c934ac/crates/uv-install-wheel/src/wheel.rs#L425)
notes:

> scripts might be binaries, so we read an exact number of bytes instead of the first line as string

Indeed, one wheel that contains a binary "script" is [`uv` itself](https://pypi.org/project/uv/).

Constructing a venv that happens to include `uv` was previously failing
with:

```
ERROR: /Users/peter/tecton/sdk/pypi/BUILD.bazel:97:8: Action sdk/pypi/.venv failed: (Exit 1): sandbox-exec failed: error executing Action command
  (cd /private/var/tmp/_bazel_peter/dfecb8ec3f6f433d8509be7ebe017232/sandbox/darwin-sandbox/589/execroot/_main && \
  exec env - \
    TMPDIR=/var/folders/9_/p2d_shr10b91_464_3jfl5t80000gn/T/ \
  /usr/bin/sandbox-exec -f /private/var/tmp/_bazel_peter/dfecb8ec3f6f433d8509be7ebe017232/sandbox/darwin-sandbox/589/sandbox.sb /var/tmp/_bazel_peter/install/96e26d97222159f904e14600d7490eb0/process-wrapper '--timeout=0' '--kill_delay=15' '--stats=/private/var/tmp/_bazel_peter/dfecb8ec3f6f433d8509be7ebe017232/sandbox/darwin-sandbox/589/stats.out' bazel-out/darwin_arm64-opt-exec-ST-2adb5a2e0ae2/bin/external/aspect_rules_py~/py/tools/venv_bin/venv_macos_aarch64_build '--location=bazel-out/darwin_arm64-fastbuild/bin/sdk/pypi/.venv' '--venv-shim=bazel-out/darwin_arm64-fastbuild-ST-2adb5a2e0ae2/bin/external/aspect_rules_py~/py/tools/venv_shim/shim_macos_aarch64_build' '--python=python_3.8_macos_aarch64_runtime/python/install/bin/python3.8' '--pth-file=bazel-out/darwin_arm64-fastbuild/bin/sdk/pypi/venv.pth' '--env-file=bazel-out/darwin_arm64-fastbuild/bin/sdk/pypi/venv.env' '--bin-dir=bazel-out/darwin_arm64-fastbuild/bin' '--collision-strategy=error' '--venv-name=.venv' '--mode=static-copy' '--version=3.8')
Error:   × Unable to run command:
  ╰─▶ stream did not contain valid UTF-8
```

Author: plobsing

py/tools/py/src/venv.rs

@@ -8,7 +8,7 @@ use sha256::try_digest;
 use std::{
     env::current_dir,
     fs::{self, File},
-    io::{BufRead, BufReader, BufWriter, Write},
+    io::{self, BufRead, BufReader, BufWriter, Read, Write, Seek},
     os::unix::fs::{MetadataExt, PermissionsExt},
     path::{Path, PathBuf},
 };
@@ -172,7 +172,34 @@ fn copy(original: &PathBuf, link: &PathBuf) -> miette::Result<()> {
     ));
 }
 
-const RELOCATABLE_SHEBANG: &str = "\
+fn copy_and_patch_shebang(original: &PathBuf, link: &PathBuf) -> miette::Result<()> {
+    let mut src = File::open(original.as_path()).into_diagnostic()?;
+
+    let mut buf = [0u8; PLACEHOLDER_SHEBANG.len()];
+    let found_shebang = match src.read_exact(&mut buf) {
+        Ok(()) => buf == PLACEHOLDER_SHEBANG,
+        Err(error) => match error.kind() {
+            io::ErrorKind::UnexpectedEof => false, // File too short to contain shebang.
+            _ => Err(error).into_diagnostic()?,
+        },
+    };
+
+    if found_shebang {
+        let mut dst = File::create(link.as_path()).into_diagnostic()?;
+        dst.write_all(RELOCATABLE_SHEBANG).into_diagnostic()?;
+        src.rewind().into_diagnostic()?;
+        io::copy(&mut src, &mut dst).into_diagnostic()?;
+    }
+    else {
+        copy(original, link)?;
+    }
+
+    Ok(())
+}
+
+const PLACEHOLDER_SHEBANG: &[u8] = b"#!/dev/null";
+
+const RELOCATABLE_SHEBANG: &[u8] = b"\
 #!/bin/sh
 '''exec' \"$(dirname -- \"$(realpath -- \"$0\")\")\"/'python3' \"$0\" \"$@\"
 ' '''
@@ -619,11 +646,7 @@ pub fn create_tree(
 
             // In the case of copying bin entries, we need to patch them. Yay.
             if link_dir.file_name() == Some(OsStr::new("bin")) {
-                let mut content = fs::read_to_string(original_entry).into_diagnostic()?;
-                if content.starts_with("#!/dev/null") {
-                    content.replace_range(..0, &RELOCATABLE_SHEBANG);
-                }
-                fs::write(&link_entry, content).into_diagnostic()?;
+                copy_and_patch_shebang(&original_entry, &link_entry)?;
             }
             // Normal case of needing to link a file :smile:
             else {