flang-aarch64-ubuntu-clang: Refine the set-up for `-WError`

As this builder is primiarly used for testing Flang, use -WError only
when building Flang (as opposed to all of LLVM).

Differential Revision: https://reviews.llvm.org/D88811
diff --git a/buildbot/osuosl/master/config/builders.py b/buildbot/osuosl/master/config/builders.py
index 0272a18..3d35bd9 100644
--- a/buildbot/osuosl/master/config/builders.py
+++ b/buildbot/osuosl/master/config/builders.py
@@ -1522,7 +1522,8 @@
                             "-DCMAKE_CXX_COMPILER=/usr/bin/clang++-10",
                             "-DLLVM_INSTALL_UTILS=ON",
                             "-DCMAKE_CXX_STANDARD=17",
-                            "-DLLVM_ENABLE_WERROR=ON",
+                            "-DLLVM_ENABLE_WERROR=OFF",
+                            "-DFLANG_ENABLE_WERROR=ON",
                             "-DBUILD_SHARED_LIBS=ON",
                             "-DLLVM_ENABLE_ASSERTIONS=ON",
                             "-DLLVM_ENABLE_LIBCXX=On",