bcl: Fix copy_bcl for desktop-win32 profile

[akien-mga]

Seems like godotengine/godot#50486 was not due to cross-compilation issues, but to the fact that windows.py copy_bcl didn't properly copy the desktop-win32 BCL, so we were using the desktop (POSIX) one.

That's why @bruvzg found that projects exported to Windows with 3.4 beta 1 worked fine, since the export workflow properly uses the net_4_x-win32 BCL.

---

Got a successful container build (will PR in a bit), currently testing the produced builds to validate.

[akien-mga]

That doesn't seem to work as is:

make: Entering directory '/home/runner/mono-configs/bcl/runtime'
if test -w /home/runner/work/godot-mono-builds/godot-mono-builds/mono_sources/mcs; then :; else chmod -R +w /home/runner/work/godot-mono-builds/godot-mono-builds/mono_sources/mcs; fi
cd /home/runner/work/godot-mono-builds/godot-mono-builds/mono_sources/mcs && make --no-print-directory -s NO_DIR_CHECK=1 PROFILES='net_4_x-win32' CC='gcc' all-profiles
build/rules.make:112: build/profiles/net_4_x-win32.make: No such file or directory

Edit: Fixed by going back to a simpler approach.

[akien-mga]

I've confirmed the fix with a rebuild of my Windows container using PR (godotengine/build-containers#86).

Testing binaries in https://downloads.tuxfamily.org/godotengine/testing/3.4.beta.20210726.mono/ (fixed-container ones, the others are ad hoc tests respectively copying the MinGW (container) and VS (GH Actions) built win32 BCLs in the right location manually).