Rollup merge of #111979 - jyn514:cargoflags, r=albertlarsan68

Respect CARGOFLAGS in bootstrap.py

This makes it possible to pass flags to bootstrap itself, for consistency with std/rustc: 04265621f9/src/bootstrap/builder.rs (L1446-L1447)
Like RUSTFLAGS, this doesn't support CARGOFLAGS_BOOTSTRAP: 6674dcda7a/src/bootstrap/bootstrap.py (L883-L884)

I found this useful recently when I wanted to pass `-Zsparse-registry` to an old checkout of the compiler from before it was stabilized in cargo.

src/bootstrap/bootstrap.py

@@ -918,6 +918,10 @@ class RustBuild(object):
             args.append("--color=always")
         elif color == "never":
             args.append("--color=never")
+        try:
+            args += env["CARGOFLAGS"].split()
+        except KeyError:
+            pass
 
         # Run this from the source directory so cargo finds .cargo/config
         run(args, env=env, verbose=self.verbose, cwd=self.rust_root)