Check that rustc can be built successfully with parallel queries for PRs.

src/ci/docker/mingw-check/Dockerfile

@@ -19,4 +19,5 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 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

src/ci/docker/x86_64-gnu-debug/Dockerfile

@@ -16,7 +16,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh
 
-ENV PARALLEL_CHECK 1
+ENV RUN_CHECK_WITH_PARALLEL_QUERIES 1
 ENV RUST_CONFIGURE_ARGS \
       --build=x86_64-unknown-linux-gnu \
       --enable-debug \

src/ci/run.sh

@@ -78,9 +78,9 @@ fi
 # sccache server at the start of the build, but no need to worry if this fails.
 SCCACHE_IDLE_TIMEOUT=10800 sccache --start-server || true
 
-if [ "$PARALLEL_CHECK" != "" ]; then
+if [ "$RUN_CHECK_WITH_PARALLEL_QUERIES" != "" ]; then
   $SRC/configure --enable-experimental-parallel-queries
-  python2.7 ../x.py check
+  CARGO_INCREMENTAL=0 python2.7 ../x.py check
   rm -f config.toml
   rm -rf build
 fi