SCons: Use default env["ENV"] and prepend PATH to it
SCons: Use default env["ENV"] and prepend PATH to it

See discussion in #46814. Now going with the safe option again (like in 3.2)
as it turns out that we can't rely on user environments on Windows, since each
shell has a different set of env variables (especially the ones necessary to
use MSVC).

SCons does its own magic when we don't pass it an `ENV` dictionary, so we
should preserve it and only add things in a second step.

Fixes this warning when compiling with MSVC using git-bash.exe:
```
Missing environment variable: WindowsSdkDir
```

Possibly fixes build issues when having both MinGW and MSVC installed and an
older SCons version.

MarianoGnu/godot,Faless/godot,honix/godot,pkowal1982/godot,josempans/godot,sanikoyes/godot,guilhermefelipecgs/godot,guilhermefelipecgs/godot,vnen/godot,pkowal1982/godot,akien-mga/godot,ZuBsPaCe/godot,sanikoyes/godot,firefly2442/godot,Shockblast/godot,akien-mga/godot,vkbsb/godot,josempans/godot,josempans/godot,honix/godot,guilhermefelipecgs/godot,akien-mga/godot,guilhermefelipecgs/godot,Valentactive/godot,Zylann/godot,Valentactive/godot,BastiaanOlij/godot,Shockblast/godot,firefly2442/godot,sanikoyes/godot,guilhermefelipecgs/godot,pkowal1982/godot,honix/godot,ZuBsPaCe/godot,firefly2442/godot,vnen/godot,Shockblast/godot,MarianoGnu/godot,BastiaanOlij/godot,BastiaanOlij/godot,Faless/godot,godotengine/godot,firefly2442/godot,Zylann/godot,DmitriySalnikov/godot,godotengine/godot,Faless/godot,pkowal1982/godot,Zylann/godot,BastiaanOlij/godot,vkbsb/godot,Zylann/godot,sanikoyes/godot,Faless/godot,Valentactive/godot,ZuBsPaCe/godot,godotengine/godot,Faless/godot,josempans/godot,BastiaanOlij/godot,Valentactive/godot,guilhermefelipecgs/godot,josempans/godot,ZuBsPaCe/godot,ZuBsPaCe/godot,DmitriySalnikov/godot,vnen/godot,Valentactive/godot,josempans/godot,ZuBsPaCe/godot,DmitriySalnikov/godot,Zylann/godot,vkbsb/godot,vkbsb/godot,akien-mga/godot,godotengine/godot,pkowal1982/godot,akien-mga/godot,akien-mga/godot,firefly2442/godot,vnen/godot,Shockblast/godot,vkbsb/godot,vnen/godot,godotengine/godot,Zylann/godot,Shockblast/godot,vkbsb/godot,sanikoyes/godot,MarianoGnu/godot,Faless/godot,BastiaanOlij/godot,MarianoGnu/godot,akien-mga/godot,sanikoyes/godot,Valentactive/godot,Shockblast/godot,godotengine/godot,MarianoGnu/godot,godotengine/godot,Zylann/godot,Faless/godot,MarianoGnu/godot,firefly2442/godot,sanikoyes/godot,honix/godot,pkowal1982/godot,josempans/godot,DmitriySalnikov/godot,vnen/godot,DmitriySalnikov/godot,BastiaanOlij/godot,ZuBsPaCe/godot,sanikoyes/godot,guilhermefelipecgs/godot,DmitriySalnikov/godot,godotengine/godot,Faless/godot,josempans/godot,vkbsb/godot,vnen/godot,vkbsb/godot,pkowal1982/godot,Valentactive/godot,MarianoGnu/godot,Shockblast/godot,honix/godot,firefly2442/godot,vnen/godot,Valentactive/godot,akien-mga/godot,ZuBsPaCe/godot,MarianoGnu/godot,Zylann/godot,DmitriySalnikov/godot,pkowal1982/godot,Shockblast/godot,honix/godot,guilhermefelipecgs/godot,BastiaanOlij/godot,firefly2442/godot