Motivation for this change

This is a flag that disables subversion keyword substitution.
Keyword substitution inserts metadata into the files being checked
out, and is therefore somewhat at odds with build reproducibility.

In particular, it can become a problem if you're trying to switch
between svn and a git export of the same thing (keyword substitutions
are normally not exported into git).

Things done

Used fetchGit and fetchSvn to fetch openttd. They produced different sha256 hashes initially, but they produce the same sha256 hash when I use ignoreKeywords

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip" (says: Nothing changed)
• (vacuously) Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.