Set a lower ThinLTO import limit for PR CI image.

Author: michaelwoerister

src/ci/docker/x86_64-gnu-llvm-7/Dockerfile

@@ -25,7 +25,9 @@ RUN sh /scripts/sccache.sh
 ENV RUST_CONFIGURE_ARGS \
       --build=x86_64-unknown-linux-gnu \
       --llvm-root=/usr/lib/llvm-7 \
-      --enable-llvm-link-shared
+      --enable-llvm-link-shared \
+      --set rust.thin-lto-import-instr-limit=10
+
 ENV SCRIPT python2.7 ../x.py test src/tools/tidy && python2.7 ../x.py test
 
 # The purpose of this container isn't to test with debug assertions and