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Flatten the INC definition to one line.

Co-authored-by: lzutao <taolzu@gmail.com>

Author: cuviper

src/ci/docker/host-x86_64/dist-x86_64-linux/build-clang.sh

@@ -18,8 +18,7 @@ cd clang-build
 # For whatever reason the default set of include paths for clang is different
 # than that of gcc. As a result we need to manually include our sysroot's
 # include path, /rustroot/include, to clang's default include path.
-INC="/rustroot/include"
-INC="$INC:/usr/include"
+INC="/rustroot/include:/usr/include"
 
 hide_output \
     cmake ../llvm \