Give a better error message for cargo check on libstd itself

Previously, the errors looked like this:

```
$ cargo check
    Checking std v0.0.0 (/home/joshua/rustc/library/std)
error: duplicate lang item in crate `core`: `bool`.
  |
  = note: the lang item is first defined in crate `core` (which `std` depends on)
  = note: first definition in `core` loaded from /home/joshua/.local/lib/cargo/target/debug/deps/libcore-afaeeb022194dcf3.rmeta
  = note: second definition in `core` loaded from /home/joshua/.local/lib/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-2675a9a46b5cec89.rlib

error[E0119]: conflicting implementations of trait `ffi::c_str::CString::new::SpecIntoVec` for type `&str`:
   --> library/std/src/ffi/c_str.rs:392:9
    |
379 |         impl<T: Into<Vec<u8>>> SpecIntoVec for T {
    |         ---------------------------------------- first implementation here
...
392 |         impl SpecIntoVec for &'_ str {
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&str`
    |
    = note: upstream crates may add a new impl of trait `core::convert::Into<alloc_crate::vec::Vec<u8>>` for type `&str` in future versions

error: aborting due to 119 previous errors; 93 warnings emitted
```

Now, they're much more helpful:

```
$ cargo check
   Compiling core v0.0.0 (/home/joshua/rustc2/library/core)
error: failed to run custom build command for `core v0.0.0 (/home/joshua/rustc2/library/core)`

Caused by:
  process didn't exit successfully: `/home/joshua/.local/lib/cargo/target/debug/build/core-ea3b18d34c1ee1c8/build-script-build` (exit code: 101)
  --- stderr
  error: you are attempting to build libcore without going through bootstrap
  help: use `x.py build --stage 0 library/std`, not `cargo build`
  help: use `x.py check`, not `cargo check`
  note: if you're sure you want to do this, use `RUSTC_BOOTSTRAP=0` or some other dummy value
  thread 'main' panicked at 'explicit panic', library/core/build.rs:7:9
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

The metric here is 'did you set RUSTC_BOOTSTRAP'; if so, it's assumed
you know what you're doing and no error is given. It's very unlikely (or
at least should be!) that someone will have RUSTC_BOOTSTRAP globally
set, so I think this is a good heuristic.

library/core/build.rs

@@ -0,0 +1,13 @@
+fn main() {
+    if !std::env::var("RUSTC_BOOTSTRAP").is_ok() {
+        eprintln!("error: you are attempting to build libcore without going through bootstrap");
+        eprintln!("help: use `x.py build --stage 0 library/std`, not `cargo build`");
+        eprintln!("help: use `x.py check`, not `cargo check`");
+        eprintln!(
+            "note: if you're sure you want to do this, use `RUSTC_BOOTSTRAP=0` or some other dummy value"
+        );
+        panic!();
+    }
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-env-changed=RUSTC_BOOTSTRAP");
+}