Use posix_spawn() on unix if program is a path

library/std/src/sys/unix/process/process_common.rs

@@ -253,11 +253,17 @@ impl Command {
         let maybe_env = self.env.capture_if_changed();
         maybe_env.map(|env| construct_envp(env, &mut self.saw_nul))
     }
+
     #[allow(dead_code)]
     pub fn env_saw_path(&self) -> bool {
         self.env.have_changed_path()
     }
 
+    #[allow(dead_code)]
+    pub fn program_is_path(&self) -> bool {
+        self.program.to_bytes().contains(&b'/')
+    }
+
     pub fn setup_io(
         &self,
         default: Stdio,

library/std/src/sys/unix/process/process_unix.rs

@@ -279,7 +279,7 @@ impl Command {
 
         if self.get_gid().is_some()
             || self.get_uid().is_some()
-            || self.env_saw_path()
+            || (self.env_saw_path() && !self.program_is_path())
             || !self.get_closures().is_empty()
         {
             return Ok(None);