Auto merge of rust-lang#56703 - alexcrichton:fix-tools, r=Mark-Simulacrum

Fix build of the `build-manifest` tool

Accidentally broken in rust-lang#56258!

Author: bors

src/ci/docker/mingw-check/Dockerfile

@@ -20,4 +20,5 @@ COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh
 
 ENV RUN_CHECK_WITH_PARALLEL_QUERIES 1
-ENV SCRIPT python2.7 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu
+ENV SCRIPT python2.7 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu && \
+           python2.7 ../x.py build --stage 0 src/tools/build-manifest

src/tools/build-manifest/src/main.rs

@@ -14,7 +14,7 @@ extern crate serde_derive;
 
 use std::collections::BTreeMap;
 use std::env;
-use std::fs::File;
+use std::fs;
 use std::io::{self, Read, Write};
 use std::path::{PathBuf, Path};
 use std::process::{Command, Stdio};