Auto merge of #3402 - RalfJung:miri-script, r=RalfJung

miri script: build with stable toolchain

`./miri toolchain` sets up a `rustup override miri`. But then if something goes wrong and the `miri` toolchain doesn't work, one can't even run `./miri toolchain` again as building miri-script needs a toolchain...

So let's always use stable to build miri-script, making it override-independent. I assume everyone will have that installed.

miri

@@ -3,5 +3,5 @@ set -e
 # Instead of doing just `cargo run --manifest-path .. $@`, we invoke miri-script binary directly. Invoking `cargo run` goes through
 # rustup (that sets it's own environmental variables), which is undesirable.
 MIRI_SCRIPT_TARGET_DIR="$(dirname "$0")"/miri-script/target
-cargo build $CARGO_EXTRA_FLAGS -q --target-dir "$MIRI_SCRIPT_TARGET_DIR" --manifest-path "$(dirname "$0")"/miri-script/Cargo.toml
+cargo +stable build $CARGO_EXTRA_FLAGS -q --target-dir "$MIRI_SCRIPT_TARGET_DIR" --manifest-path "$(dirname "$0")"/miri-script/Cargo.toml
 "$MIRI_SCRIPT_TARGET_DIR"/debug/miri-script "$@"

miri.bat

@@ -5,7 +5,7 @@ set MIRI_SCRIPT_TARGET_DIR=%0\..\miri-script\target
 
 :: If any other steps are added, the "|| exit /b" must be appended to early
 :: return from the script. If not, it will continue execution.
-cargo build %CARGO_EXTRA_FLAGS% -q --target-dir %MIRI_SCRIPT_TARGET_DIR% --manifest-path %0\..\miri-script\Cargo.toml || exit /b
+cargo +stable build %CARGO_EXTRA_FLAGS% -q --target-dir %MIRI_SCRIPT_TARGET_DIR% --manifest-path %0\..\miri-script\Cargo.toml || exit /b
 
 :: Forwards all arguments to this file to the executable.
 :: We invoke the binary directly to avoid going through rustup, which would set some extra