…bey overrides of use_mingw

Let the platforms optionally override the default scons environment tools
via a `custom_tools` flag, so the default SConstruct only needs to set the
defaults and not worry about each individual platform requirements.
Users can also override the tools via `custom_tools`.

Add a temporary SCons environment to only process `use_mingw`, `platform`,
`target` and `custom_tools` options from all the different places where
they can be set, and then use those values to initialize the actual
environment that we use to build, as well as determine which builder scripts
to load based on the target.

This fixes a related issue where, if a platform sets a default target
in `get_flags` different from editor, and the user doesn't pass in the
target in the command line, the `editor` target would be briefly selected
during a portion of SConstruct execution, long enough to set the
env.editor_build to true, causing editor files and defines to be set
for non-editor builds.

Fixes godotengine#60719

Make sure that if the user sets a value equal to the default and the
platform sets a value different from the default, that the user value
is respected.