SCons: Fix build with `p` alias or platform auto-detection

Fixes a pre-existing bug that #44433 exposed.

It's pretty hacky, but we use `platform` in `env` both as an optional command
line option (instead it can be autodetected, or passed via the `p` alias, and
on Linux it might be overridden if you pass one of the convenience alias
values), and as the reference value for what platform we're building on.

Thus we override `env_base["platform"]` with the autodetected or validated
platform, but any call to `opts.Update(env_base)` overrides it with the
original command line option... causing e.g. #44448.

The proper fix would be to refactor all this so that we don't reuse
`env["platform"]` for platform detection (it could instead be e.g.
`env.platform` as a member variable which holds the validated value),
but for now I'm tapering over the immediate breakage.

Fixes #44448 and other breakages induced by #44433.

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