When `build_nim.sh` detects that `skipIntegrityCheck` is supported,
it runs the upstream toolchain for building Nim, ignoring the locally
checked out `Nim-csources-v1` and `nimble` sub-repositories. So, having
those repos checked out is not useful in that scenario.
When `skipIntegrityCheck` is unsupported, `build_nim.sh` uses the local
sub-repositories if they are available. But, it also clones them if they
are unavailable using commits as specified by environment variables.
Because the clone will happen as part of the script, having the repos
linked as sub-repositories is also not useful. Even worse, if the script
has different commit hashes than the subrepository, the behaviour is
different depending on whether `build_nim.sh` is run before or after
the submodules are checked out.
Therefore, we can remove the `Nim-csources-v1` and `nimble` sub-repos,
using the upstream build system if available, or the hardcoded commits
inside `build_nim.sh` to have a manual build, as before.
Also moves the various variables that control the used versions into
the manual section to emphasize that these only take effect in manual
mode, but not when using upstream build system (`skipIntegrityCheck`).
If one wants to use a custom `Nimble` version, fork `Nim` and adjust the
config in that fork. That also works with the upstream build system.
3866a8ab98