Auto merge of #1417 - RalfJung:libstd-rebuilds, r=RalfJung

fix libstd rebuilds due to RUSTFLAGS changes

Until recently we were always overwriting `RUSTFLAGS`, but we stopped doing that when moving to `MIRI_BE_RUSTC`. That introduced the regression in #1410. This fixes it.

Fixes #1410

Author: bors

src/bin/cargo-miri.rs

@@ -335,6 +335,7 @@ path = "lib.rs"
     command.current_dir(&dir);
     command.env("XARGO_HOME", &dir);
     command.env("XARGO_RUST_SRC", &rust_src);
+    command.env_remove("RUSTFLAGS"); // Make sure external `RUSTFLAGS` do not influence the build.
     // Use Miri as rustc to build a libstd compatible with us (and use the right flags).
     // However, when we are running in bootstrap, we cannot just overwrite `RUSTC`,
     // because we still need bootstrap to distinguish between host and target crates.