iwyu.yml: remove duplicated option passed to cmake (#6213)

for cmake, the last option win, so there is no need to repeat the same
option multiple times. but we have two `-DBUILD_TESTS=On` passed to
cmake.

in this change, one of them is dropped.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

Author: tchaikov

.github/workflows/iwyu.yml

@@ -151,7 +151,7 @@ jobs:
 
       - name: Prepare CMake
         run: |
-          cmake -S . -B cmake.output -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DHAVE_RULES=On -DBUILD_TESTS=On -DBUILD_GUI=On -DBUILD_TESTS=On -DBUILD_GUI=On -DWITH_QCHART=On -DENABLE_CHECK_INTERNAL=On -DCMAKE_GLOBAL_AUTOGEN_TARGET=On -DCMAKE_DISABLE_PRECOMPILE_HEADERS=On -DCPPCHK_GLIBCXX_DEBUG=Off -DUSE_MATCHCOMPILER=Off -DEXTERNALS_AS_SYSTEM=On
+          cmake -S . -B cmake.output -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DHAVE_RULES=On -DBUILD_TESTS=On -DBUILD_GUI=On -DBUILD_GUI=On -DWITH_QCHART=On -DENABLE_CHECK_INTERNAL=On -DCMAKE_GLOBAL_AUTOGEN_TARGET=On -DCMAKE_DISABLE_PRECOMPILE_HEADERS=On -DCPPCHK_GLIBCXX_DEBUG=Off -DUSE_MATCHCOMPILER=Off -DEXTERNALS_AS_SYSTEM=On
         env:
           CC: clang-18
           CXX: clang++-18
@@ -176,4 +176,4 @@ jobs:
       - uses: actions/upload-artifact@v3
         with:
           name: Logs (clang-include-cleaner)
-          path: ./*.log
+          path: ./*.log