feature: `choosenim csources v0.19.0` (for bootstrapping older nims during bisect)
see design idea discussed here: https://github.com/nim-lang/Nim/issues/13066#issuecomment-572262660
the idea is to allow automatically building nim at **any** prior commit (eg use case: for `git bisect`) and using an appropriate csources-built nim version as bootstrapping point is guaranteed to work, unlike any of the releases built by choosenim. For example (that occurred while bisecting for https://github.com/nim-lang/Nim/issues/13066):

```
git checkout 9985706fb812156fa6f805989063293ca71599df
choosenim 0.19.0 # ditto with 0.19.2 0.19.4 0.20.0
nim c -o:bin/nim_temp1 -d:release --skipParentCfg --skipUserCfg compiler/nim.nim
pathutils.nim(13, 22) Error: cannot open file: pathnorm
```
whereas it works if we build from csources at tag v0.19.0

