.github: use Debug build for header dependency verification

The existing GitHub workflow uses `RelWithDebInfo` configuration when
running clang-include-cleaner, which can miss header dependencies
conditionally compiled with SEASTAR_DEBUG.

This change switches the workflow to use Debug configuration to ensure:
- Proper detection of headers guarded by `SEASTAR_DEBUG`
- Correct verification of conditional include statements
- Accurate identification of truly unused headers

Specifically, this addresses the false-positive detection of
utils/assert.hh as an unused header, which is conditionally included
when SEASTAR_DEBUG is defined.

Signed-off-by: Kefu Chai <[email protected]>

Author: tchaikov

.github/workflows/iwyu.yaml

@@ -6,7 +6,7 @@ on:
       - master
 
 env:
-  BUILD_TYPE: RelWithDebInfo
+  BUILD_TYPE: Debug
   BUILD_DIR: build
   CLEANER_OUTPUT_PATH: build/clang-include-cleaner.log
   # the "idl" subdirectory does not contain C++ source code. the .hh files in it are