make sure the lock file is not changed when building on CI

Author: RalfJung

.travis.yml

@@ -16,7 +16,7 @@ script:
   xargo/build.sh
 - |
   # Test plain miri
-  cargo build --release --features "cargo_miri" &&
+  cargo build --locked --release --features "cargo_miri" &&
   cargo test --release --all &&
   cargo install --features "cargo_miri"
 - |
@@ -31,7 +31,7 @@ script:
 - |
   # test that the rustc_tests binary compiles
   cd rustc_tests &&
-  cargo build --release &&
+  cargo build --locked --release &&
   cd ..
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