If there was no older file than $NIX_BUILD_TOP this would result in a
warning, e.g. with nix-info.
```
/nix/store/15kgcm8hnd99p7plqzx7p4lcr2jni4df-set-source-date-epoch-to-latest.sh: line 13: [: : integer expression expected
```
set-source-date-epoch-to-latest.sh to ignore files newer than "$NIX_BUILD_TOP/.." (unlike "$NIX_BUILD_TOP" it is root-owned and cannot be touched by nixbld1).
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.
