stdenv: trim random seed to avoid reference cycles

Using the full store hash as the random seed occasionally caused
reference cycles when the invocation was stored in output artifacts.
For example, cross-compiled gcc was failing due to this:
https://hydra.nixos.org/eval/1631713#tabs-now-fail

Simply truncating the hash is sufficient to avoid this.

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

@@ -1,4 +1,9 @@
 # Use the last part of the out path as hash input for the build.
 # This should ensure that it is deterministic across rebuilds of the same
 # derivation and not easily collide with other builds.
-export NIX_CFLAGS_COMPILE+=" -frandom-seed=${out##*/}"
+# We also truncate the hash so that it cannot cause reference cycles.
+export NIX_CFLAGS_COMPILE+=" -frandom-seed=$(
+    outbase="${out##*/}"
+    randomseed="${outbase:0:10}"
+    echo $randomseed
+)"