fix(CI): bootstrap.py no longer reuses a stale package database

[zlonast]

fix: #12311

The package database is now deleted and recreated from scratch on every
bootstrap run, making the script idempotent with respect to a dirty or
polluted _build. Additionally, the Bootstrap workflow now hashes the
.cabal files of the local packages into its cache key, so caches are no
longer shared between branches that build different versions of the local
packages.

  $ python3 bootstrap/bootstrap.py --bootstrap-sources bootstrap-sources.tar.gz
+ Deleting stale package database _build/packages.conf
  Creating package database _build/packages.conf
  Using ...

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Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[ulysses4ever]

Sounds good to me.