guix: Adjust for Qt 6

1. Do not set `C{PLUS}_INCLUDE_PATH` variables

The build system for Qt 6 differs entirely from that of Qt 5. Building a
set of native Qt 6 tools now forms a separate step when cross-compiling.
Under these new circumstances, the `C{PLUS}_INCLUDE_PATH` environment
variables may alter the default include directories for both native and
cross compilers.

Previously, we explicitly unset these variables when invoking clang for
cross-compiling; however, that approach proved suboptimal (see #30451).

This change sets the native toolchain for dependencies explicitly,
rather than relying on the `C{PLUS}_INCLUDE_PATH` environment variables.
Additionally, it facilitates the transition towards using clang for
building native tools when cross-compiling for macOS.

2. Add `ninja` package.

3. Adjust allowed symbol lists.

contrib/guix/manifest.scm

@@ -13,6 +13,7 @@
              ((gnu packages linux) #:select (linux-libre-headers-6.1))
              (gnu packages llvm)
              (gnu packages mingw)
+             (gnu packages ninja)
              (gnu packages pkg-config)
              ((gnu packages python) #:select (python-minimal))
              ((gnu packages python-build) #:select (python-tomli python-poetry-core))
@@ -547,6 +548,7 @@ inspecting signatures in Mach-O binaries.")
         gcc-toolchain-13
         cmake-minimal
         gnu-make
+        ninja
         ;; Scripting
         python-minimal ;; (3.10)
         ;; Git