setup-hooks/reproducible-builds.sh: NIX_OUTPATH_USED_AS_RANDOM_SEED

For reproducibility, nixpkgs sets `-frandom-seed` to the first 10
characters of the outpath of the derivation being built.

This PR allows to manually select from which outpath the
10-character prefix is taken, by setting
`NIX_OUTPATH_USED_AS_RANDOM_SEED` as a derivation attribute (or
directly as an environment variable).

For an example use of this functionality, see:

  https://github.com/NixOS/nixpkgs/pull/209870

pkgs/build-support/setup-hooks/reproducible-builds.sh

@@ -3,7 +3,8 @@
 # derivation and not easily collide with other builds.
 # We also truncate the hash so that it cannot cause reference cycles.
 NIX_CFLAGS_COMPILE="${NIX_CFLAGS_COMPILE:-} -frandom-seed=$(
-    outbase="${out##*/}"
+    randSeed=${NIX_OUTPATH_USED_AS_RANDOM_SEED:-$out}
+    outbase="${randSeed##*/}"
     randomseed="${outbase:0:10}"
     echo $randomseed
 )"