ci: Don't fetch opencv submodule unless on windows

.ci/build-linux.sh

@@ -6,10 +6,10 @@ fi
 
 git config --global --add safe.directory '*'
 
-# Pull all the submodules except llvm
+# Pull all the submodules except llvm and opencv
 # Note: Tried to use git submodule status, but it takes over 20 seconds
 # shellcheck disable=SC2046
-git submodule -q update --init $(awk '/path/ && !/llvm/ { print $3 }' .gitmodules)
+git submodule -q update --init $(awk '/path/ && !/llvm/ && !/opencv/ { print $3 }' .gitmodules)
 
 mkdir build && cd build || exit 1