SCons: Add `fast_unsafe` option for faster rebuilds

This reverts #53828 which had caused a significant drop in incremental
rebuild time for debug builds (from 10s to 23s on my laptop).

The "faster but unsafe" options are re-added, as well as adding
`max_drift=60` which we didn't use previously.

These options speed up SCons' own processing of the codebase to decide
what to build/rebuild (i.e. the first step before actually calling the
compiler). This will therefore not make much difference for scratch
builds, and is mostly useful for incremental rebuilds (including "null"
rebuilds with no change).

These options are enabled automatically for `debug` builds, unless
`fast_unsafe=no` is passed.
They are disabled by default for `release` and `release_debug` builds,
unless `fast_unsafe=yes` is passed.

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