Improve error when a compiler/library build fails in checktools.sh

Author: Kobzol

src/ci/docker/host-x86_64/x86_64-gnu-tools/checktools.sh

@@ -8,6 +8,10 @@ X_PY="$1"
 
 # Try to test the toolstate-tracked tools and store the build/test success in the TOOLSTATE_FILE.
 
+# Pre-build the compiler and the library first to output a better error message when the build
+# itself fails (see https://github.com/rust-lang/rust/issues/127869 for context).
+python3 "$X_PY" build --stage 2 compiler rustdoc
+
 set +e
 python3 "$X_PY" test --stage 2 --no-fail-fast \
     src/doc/book \