guix: use clang-toolchain-15 for macOS compilation

Author: fanquake

contrib/guix/manifest.scm

@@ -610,5 +610,5 @@ inspecting signatures in Mach-O binaries.")
           ((string-contains target "-linux-")
            (list (make-bitcoin-cross-toolchain target)))
           ((string-contains target "darwin")
-           (list clang-toolchain-11 binutils cmake-minimal xorriso python-signapple))
+           (list clang-toolchain-15 binutils cmake-minimal xorriso python-signapple))
           (else '())))))