Node and friends use --random-seed to temporary reset the seed for
predictable code-cache creation. To allow custom random seeds at runtime
the flag is reset for encoding the FlagList::Hash in the snapshots.

We will soon disallow changing flags via the API after V8 has been
initialized. In order to make node work we will exclude --random-seed
from the FlagList::Hash calculation.

Drive-by-fix:
* Lazily initialize flag_hash instead of calculating it after every call
  to SetFlagsFromString / EnforceFlagImplications.
* Simplify hash string source creation since out << flag now includes
  the full flag information

Bug: v8:12309
Change-Id: I1a168f4702d8c4d160ff12fdbea881731e4ea8b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3218159Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77345}