build: do not use mode as the index into modes

before this change, in `configure_seastar()`, we use `mode` as
a component in the build directory, and use it as the index into `modes`
dict. but in a succeeding commit, we will reuse `configure_seastar()`
when preparing for the CMake-based building system, in which,
`mode` will be the CMake configure type, like "Debug" instead of
scylla's build mode, like "debug". to be prepared for this change,
let's use `mode_config` directly. it's identical to `modes[mode]`.

this also improves the readability.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

configure.py

@@ -1722,7 +1722,7 @@ def configure_seastar(build_dir, mode, mode_config):
         seastar_cmake_args += ['-DSeastar_ALLOC_FAILURE_INJECTION=ON']
     if args.seastar_debug_allocations:
         seastar_cmake_args += ['-DSeastar_DEBUG_ALLOCATIONS=ON']
-    if modes[mode]['build_seastar_shared_libs']:
+    if mode_config['build_seastar_shared_libs']:
         seastar_cmake_args += ['-DBUILD_SHARED_LIBS=ON']
 
     seastar_cmd = ['cmake', '-G', 'Ninja', real_relpath(args.seastar_path, seastar_build_dir)] + seastar_cmake_args