Toolchain directory layout must be computed in a more flexible way on Unix-like platforms

[jakepetroules]

For example, in Fedora 43:

# swift build --build-system swiftbuild
error: Unexpected toolchain layout for Swift installation path: Path(_str: "/usr/libexec/swift/6.2/bin")
error: missing required default toolchain
error: Could not initialize build system

main branch will have the same issue. The install prefix is completely configurable and any given Linux distribution could theoretically put it anywhere, so we need to be able to derive these paths without hardcoding specific prefixes.