Bundled protoc only if the interpreter exists (#459)

Otherwise on systems such as NixOS the prost-build attempts to execute
the bundled protoc, hits a very obscure error about file not existing
and exits, without trying the other options (such as the protoc from
path, if it exists there).

Co-authored-by: Lucio Franco <luciofranco14@gmail.com>

prost-build/build.rs

@@ -35,13 +35,26 @@ fn env_protoc() -> Option<PathBuf> {
     Some(protoc)
 }
 
+/// We can only use a bundled protoc if the interpreter necessary to load the binary is available.
+///
+/// The interpreter is specific to the binary and can be queried via e.g. `patchelf
+/// --print-interpreter`, or via readelf, or similar.
+fn is_interpreter(path: &'static str) -> bool {
+    // Here we'd check for it being executable and other things, but for now it being present is
+    // probably good enough.
+    std::fs::metadata(path).is_ok()
+}
+
 /// Returns the path to the bundled `protoc`, if it is available for the host platform.
-#[cfg(not(target_env = "musl"))]
 fn bundled_protoc() -> Option<PathBuf> {
     let protoc_bin_name = match (env::consts::OS, env::consts::ARCH) {
-        ("linux", "x86") => "protoc-linux-x86_32",
-        ("linux", "x86_64") => "protoc-linux-x86_64",
-        ("linux", "aarch64") => "protoc-linux-aarch_64",
+        ("linux", "x86") if is_interpreter("/lib/ld-linux.so.2") => "protoc-linux-x86_32",
+        ("linux", "x86_64") if is_interpreter("/lib64/ld-linux-x86-64.so.2") => {
+            "protoc-linux-x86_64"
+        }
+        ("linux", "aarch64") if is_interpreter("/lib/ld-linux-aarch64.so.1") => {
+            "protoc-linux-aarch_64"
+        }
         ("macos", "x86_64") => "protoc-osx-x86_64",
         ("macos", "aarch64") => "protoc-osx-x86_64", // will be translated to aarch64 by Rosetta
         ("windows", _) => "protoc-win32.exe",
@@ -51,14 +64,6 @@ fn bundled_protoc() -> Option<PathBuf> {
     Some(bundle_path().join(protoc_bin_name))
 }
 
-/// `musl` build hosts do not have a bundled `protoc`.
-///
-/// Note: this checks the target of the `prost-build` build.rs, which is ultimately the host architecture.
-#[cfg(target_env = "musl")]
-fn bundled_protoc() -> Option<PathBuf> {
-    None
-}
-
 /// Returns the path to the `protoc` included on the `PATH`, if it exists.
 fn path_protoc() -> Option<PathBuf> {
     which::which("protoc").ok()