lang/nim: Fix the build of lang/nim within poudriere:

• Make sure we use specified build caches for all build commands (this was causing the poudriere testport to fail for the tools build).
• Ensure that devel/nimble is not built as part of this port by using the koch toolsToNimble command instead of koch tools. If left to it's own devices the koch tools command will download nimble from git and build it itself.

PR: 245789
Submitted by: Neal Nelson
Reviewed by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D24585