Fix fail to build in sbuild

nim doesn't build in sbuild/salsa pipeline. Fix that by:

• Remove nimgrep and its manpage until libpcre (pcre3) use is patched upstream
• Patch out calls to git clone in koch doc
• Change build step of koch tools to more granular calls to avoid more git clone

Builds successfully in sbuild and salsa (if you ignore piuparts/reprotests)