Auto merge of #39728 - eddyb:vendeur-tres-bien, r=alexcrichton

Automate vendoring by invoking cargo-vendor when building src dist tarballs.

This avoids #39633 bringing the `src/vendor` checked into git by #37524, past 200,000 lines of code.

I believe the strategy of having rustbuild run `cargo vendor` during the `dist src` step is sound.

However, the only way to be sure `cargo-vendor` exists is to run `cargo install --force cargo-vendor`, which will recompile it every time (not passing `--force` means you can't tell between "already exists" and "build error"). ~~This is quite suboptimal and I'd like to somehow do it in each `Dockerfile` that would need it.~~

* [ ] Cache `CARGO_HOME` (i.e. `~/.cargo`) between CI runs
  * `bin/cargo-vendor` and the actual caches are the relevant bits
* [x] Do not build `cargo-vendor` all the time
  * ~~Maybe detect `~/.cargo/bin/cargo-vendor` already exists?~~
  * ~~Could also try to build it in a `Dockerfile` but do we have `cargo`/`rustc` there?~~
  * Final solution: check `cargo install --list` for a line starting with `cargo-vendor `

cc @rust-lang/tools

src/bootstrap/bootstrap.py

@@ -294,6 +294,8 @@ class RustBuild(object):
             raise Exception("no cargo executable found at `%s`" % self.cargo())
         args = [self.cargo(), "build", "--manifest-path",
                 os.path.join(self.rust_root, "src/bootstrap/Cargo.toml")]
+        if self.use_locked_deps:
+            args.append("--locked")
         if self.use_vendored_sources:
             args.append("--frozen")
         self.run(args, env)
@@ -455,6 +457,9 @@ def main():
     rb.use_vendored_sources = '\nvendor = true' in rb.config_toml or \
                               'CFG_ENABLE_VENDOR' in rb.config_mk
 
+    rb.use_locked_deps = '\nlocked-deps = true' in rb.config_toml or \
+                         'CFG_ENABLE_LOCKED_DEPS' in rb.config_mk
+
     if 'SUDO_USER' in os.environ and not rb.use_vendored_sources:
         if os.environ.get('USER') != os.environ['SUDO_USER']:
             rb.use_vendored_sources = True