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.

pkgs/build-support/setup-hooks/set-source-date-epoch-to-latest.sh

···
 postUnpackHooks+=(_updateSourceDateEpochFromSourceRoot)
 
 _updateSourceDateEpochFromSourceRoot() {
-    updateSourceDateEpoch "$sourceRoot"
+    if [ -n "$sourceRoot" ]; then
+        updateSourceDateEpoch "$sourceRoot"
+    fi
 }