Inherit $RUSTUP_HOME

Summary:
Without this, it is impossible to run the prelude-provided `system_rust_toolchain` on systems with a Rustup-managed `rustc` in a non-default location. This includes official Microsoft devcontainers such as <https://hub.docker.com/r/microsoft/devcontainers-rust> which use `RUSTUP_HOME=/usr/local/rustup`.

```lang=text,counterexample
Action failed: root//third-party:unicode-ident-1.0.14 (rustc diag)
Local command returned non-zero exit code 1

error: rustup could not choose a version of rustc to run, because one wasn't specified explicitly, and no default is configured.

help: run 'rustup default stable' to download the latest stable release of Rust and set it as your default toolchain.

Stack backtrace:
   0: anyhow::error::<impl core::convert::From<E> for anyhow::Error>::from
   1: rustup::config::Cfg::find_or_install_override_toolchain_or_default
   2: rustup::cli::proxy_mode::main
   3: rustup_init::main
   4: std::sys_common::backtrace::__rust_begin_short_backtrace
   5: main
   6: <unknown>
   7: __libc_start_main
   8: <unknown>
```

Reviewed By: zertosh

Differential Revision: D68675864

fbshipit-source-id: 98d4685b8f59fc6304b8665115fe1027c942f74b

rust/tools/rustc_action.py

@@ -36,6 +36,7 @@
     "PATH",
     "PWD",
     "HOME",
+    "RUSTUP_HOME",
     "TMPDIR",
     # Required on Windows
     "LOCALAPPDATA",