On macOS, the build of libxxhash is performed by a bare `make`
launched by `MakePackage`s default `build` phase. As `prefix`
is not set at this point, the library gets compiled with a hard-coded
install name of `/usr/local/libxxhash.dylib`. Downstream clients,
in the tested case Root, will then fail to build as the install name
is incorrect.
Move setting of `prefix` from `install` to `edit` so that it
propagates to subsequent `build` and `install` phases.
fb6a2b5cab