lang/rust: enslave lang/rust-nightly

• rust: drop no longer used RUST_SOURCE variable
• rust: apply rust-nightly FIXME comment about stage-qa (strip)
• rust-nightly: properly conflict for install with rust package
• rust-nightly: inherit |make test| support from r401025
• rust-nightly: as DOCSDIR is the same drop unnecessary DOCS option

Approved by: riggs, dumbbell (maintainers)
Differential Revision: https://reviews.freebsd.org/D5789