Export EXTRA_CMAKE_FLAGS to remove the "unbound variable" error (#2041)

BUG=oss-fuzz:67018

tests/oss-fuzz/build.sh

diff --git a/tests/oss-fuzz/build.sh b/tests/oss-fuzz/build.sh
index 36e6278..cd7ca9e 100755
--- a/tests/oss-fuzz/build.sh
+++ b/tests/oss-fuzz/build.sh
@@ -75,6 +75,8 @@
 then
   export CXXFLAGS="${CXXFLAGS} -DFUZZTEST_COMPATIBILITY_MODE"
   export EXTRA_CMAKE_FLAGS="-DAVIF_ENABLE_FUZZTEST=ON -DFUZZTEST_COMPATIBILITY_MODE=libfuzzer"
+else
+  export EXTRA_CMAKE_FLAGS=""
 fi
 cmake .. -G Ninja -DBUILD_SHARED_LIBS=OFF -DAVIF_CODEC_AOM=LOCAL -DAVIF_CODEC_DAV1D=LOCAL \
       -DAVIF_CODEC_AOM_DECODE=ON -DAVIF_CODEC_AOM_ENCODE=ON \