feat(cargo-run): inherit jobserver from env

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 tries to provide such an ability.

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/ops/cargo_run.rs

@@ -104,6 +104,10 @@ pub fn run(
         process.display_env_vars();
     }
 
+    if let Some(client) = config.jobserver_from_env() {
+        process.inherit_jobserver(client);
+    }
+
     config.shell().status("Running", process.to_string())?;
 
     process.exec_replace()

tests/testsuite/jobserver.rs

@@ -134,11 +134,10 @@ all:
         )
         .build();
 
-    p.process(make).env("CARGO", cargo_exe()).arg("-j2")
-        .with_status(2)
-        .with_stderr_contains("[..]no jobserver from env[..]")
-        .run();
+    // jobserver can be inherited from env
+    p.process(make).env("CARGO", cargo_exe()).arg("-j2").run();
 
+    // but not from `-j` flag
     p.cargo("run -j2")
         .with_status(101)
         .with_stderr_contains("[..]no jobserver from env[..]")