feat: inherit jobserver from env for all kinds of runner

External subcommands are already able to inherit the jobserver from
env since rust-lang#10511. However, users reported that they've expected
`cargo run` to behave the same as external subcommands.

A popular example "cargo-xtask" pattern is used as scripting to run
arbitrary tasks. Users may want to invoke `cargo run` from Make and
expect some parallelism. This PR provides such an ability to the
general `target.<...>.runner`, which affects `cargo test`,
`cargo bench`, and `cargo run`.

Note that this PR doesn't create any jobserver client if there is no
existing jobserver from the environment. Nor `-j`/`--jobs` would create
a new client. Reasons for this decision:

* There might be crates don't want the jobserver to pollute their
  file descriptors, although they might be rare
* Creating a jobsever driver with the new FIFO named pipe style is not
  yet supported as of `jobserver@0.1.26`. Once we can create a named
  pipe-based jobserver, it will be less risky and inheritance by
  default can be implemented.

src/cargo/core/compiler/compilation.rs

@@ -257,7 +257,13 @@ impl<'cfg> Compilation<'cfg> {
         } else {
             ProcessBuilder::new(cmd)
         };
-        self.fill_env(builder, pkg, script_meta, kind, false)
+        let mut builder = self.fill_env(builder, pkg, script_meta, kind, false)?;
+
+        if let Some(client) = self.config.jobserver_from_env() {
+            builder.inherit_jobserver(client);
+        }
+
+        Ok(builder)
     }
 
     /// Prepares a new process with an appropriate environment to run against

tests/testsuite/jobserver.rs

@@ -192,33 +192,25 @@ test-runner:
         .env("CARGO", cargo_exe())
         .arg("run")
         .arg("-j2")
-        .with_status(2)
-        .with_stderr_contains("[..]no jobserver from env[..]")
         .run();
     p.process(make)
         .env("PATH", path)
         .env("CARGO", cargo_exe())
         .arg("run-runner")
         .arg("-j2")
-        .with_status(2)
         .with_stderr_contains("[..]this is a runner[..]")
-        .with_stderr_contains("[..]no jobserver from env[..]")
         .run();
     p.process(make)
         .env("CARGO", cargo_exe())
         .arg("test")
         .arg("-j2")
-        .with_status(2)
-        .with_stdout_contains("[..]no jobserver from env[..]")
         .run();
     p.process(make)
         .env("PATH", path)
         .env("CARGO", cargo_exe())
         .arg("test-runner")
         .arg("-j2")
-        .with_status(2)
         .with_stderr_contains("[..]this is a runner[..]")
-        .with_stdout_contains("[..]no jobserver from env[..]")
         .run();
 
     // but not from `-j` flag