v.builder: fix build using msvc with multiple C sources (for example programs using #flag file.c)

[crackedmind]

Fix #22376

[spytheman]

Please also add a test case in vlib/v/gen/c/testdata/ - a small .vv program, that uses
#flag file.c, having something like:

int answer() { return 42 }

then calls println(C.answer()), and a corresponding .out file to the .vv program, containing just a line with 42 .

[crackedmind]

and how i should do this ? i need example :)

[spytheman]

Only sanitized CI jobs remain, the Windows ones passed, so it is good to merge.