save a backup of csources-built nim to bin/nim_csources to avoid recompiling from csources

[timotheecour]

keeping the nim build from csources is useful to avoid having to rebuild it via csources/build.sh whn it's needed;
it's needed eg to make sure bootstrap still works, see for eg #8577 where problem only occurs during bootstrap, not once a bin/nim is already bootstrapped)

it also makes it more efficient to rerun sh build_all.sh if user wants to rebuild nim+toosl after hacking on compiler

[Araq]

I'll accept this but please keep the shell scripting to a minimum, I don't want to review a never ending list of "improvements" to this, that then needs to be reproduced in the .bat files for Windows support. And fixed too because it actually turned into a Bash script, not a Posix shell script. Shell scripting is programming in an error prone, unportable hard-to-read, untyped archaic language. We have koch and NimScript to automate these processes.

[timotheecour]

agreed, I also prefer nim (etc) to shell scripts in general ;
since this is for bootstrapping, there is a minimum necessary scripting to do before a nim compiler is even available to actually run nim code though; I don't expect this'll lead to a never ending list of improvements.

as for windows support, writing #8564 was painful enough (windows bat is truly aweful) so I just kept it to a minimum that allows me to automate building nim from source on windows (so i can debug windows issues too)

good to merge?