stdenv: fix set-source-date-epoch-to-latest (close #12602)

In some cases the $sourceRoot is missing. Skip the hook instead
of showing the following cryptic error:

    find: cannot search `': No such file or directory
    /nix/store/0p1afvl8jcpi6dvsq2n58i90w9c59vz1-set-source-date-epoch-to-latest.sh: line 12: [: : integer expression expected

vcunat removed the warning; the hook will just skip silently in these cases.
Perhaps someone can improve on it some time.