SCons: Fixes for warn/error colorization

[Repiteo]

Fixes a handful of regressions from #91220:

• mingw no longer fails when trying to output in a non-verbose environment
• compilation timer now uses explicit gray coloration (previously showed up as black on GitHub Actions)

[Calinou]

• compilation timer now uses explicit gray coloration (previously showed up as black on GitHub Actions)

I'm disappointed that GitHub Actions doesn't act like other terminal emulators here 🙁
"bright black" (\e[90m) shows up as gray in most terminals.

Using a 256-color gray works, but it also means that it may be harder to read on a light theme, since the user can't adjust its appearance anymore (not in most terminal emulators at least). When using 256 colors without a background color defined, make sure the text remains readable on both dark and light theme.

[Repiteo]

I went with the 50% gray for that reason; seems to show up fine on GitHub & is roughly as visible on VSCode as the old "black" text.

[Calinou]

Tested locally, it works as expected (Linux).

[akien-mga]

Thanks!