Resolve Apple toolchain paths without xcrun when developer dir is set

Apple compile, archive, merge, and code signing actions now invoke their tools by absolute path. When a target pins xcode_developer_dir, paths for swiftc, clang, clang++, libtool, lipo, and codesign are derived directly from the Xcode developer directory layout, so the cached action argv no longer contains xcrun. Toolchain path discovery and xcode_developer_dir continue to partition the action cache, and signing still uses xcrun --find codesign so a custom install on PATH is honored.