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.
This adds -frandom-seed to each compiler invocation in stdenv. The
object here is to make the compierl invocations produce the same output
every time they are called (for the same derivation). When the
-frandom-seed option is not set the compiler will use a combination of
random numbers (in GCC's case from /dev/urandom) and the durrent time to
produce a "random" input per file. This can (among other things) lead to
different ordering of symbols in the produced object files.
For reason of reproducibility we prefer having the same derivation
produce the exact same outputs. This is not a silver bullet but one way
to tame the compiler.
