xcbuild: fix sdk dir name issue

Previously, this error was coming up in xcbuild:

ld: malformed 32-bit x.y.z version number: 1068638

It’s an interesting error because it only happens with certain hashes for the
nixpkgs sdk. For instance, on latest nixpkgs unstable channel, the hash for the
xcbuild sdk is:

/nix/store/w6mwbdaz9calyii0fyxspl51f1068638-nix.nixpkgs.sdk

that is an issue we pass -isysroot ${sdk} to clang where it will interpret that
hanging "1068638". It would probably go away as soon as the hash changes but
this hacky fix will solve the problem.