Apply patch to use proper clang

cmake-fix.patch

@@ -0,0 +1,19 @@
+Commit ID: 4519c06ca33e1251b166e4d9c3ff4b63f0042a7f
+Change ID: ovkxootrstkyyzosmnlxpspwoznsoynr
+Author   : Aleix Boné <aleix.boneribo@bsc.es> (2026-02-24 12:34:33)
+Committer: Aleix Boné <aleix.boneribo@bsc.es> (2026-02-24 12:34:33)
+
+    (no description set)
+
+diff --git a/cmake/LLVMHelpers.cmake b/cmake/LLVMHelpers.cmake
+index db1a7f142e..11a0915ee2 100644
+--- a/cmake/LLVMHelpers.cmake
++++ b/cmake/LLVMHelpers.cmake
+@@ -41,7 +41,6 @@
+     "${PROG_NAME}${CMAKE_EXECUTABLE_SUFFIX}"
+     HINTS ${LOCATION}
+     DOC "${DOCSTRING}"
+-    NO_DEFAULT_PATH
+     NO_CMAKE_PATH
+     NO_CMAKE_ENVIRONMENT_PATH
+   )

default.nix

@@ -45,6 +45,8 @@ llvmPackages.stdenv.mkDerivation (finalAttrs: {
   version = "nightly";
   inherit src;
 
+  patches = [ ./cmake-fix.patch ];
+
   cmakeFlags = [
     # TODO: all these are broken when cross compiling. Upstream has refactored
     # all the cmake infra for cross compilation, but it's not in a release yet