lang/rust-nightly: skip optional tools on bustage

As r488649 and r489131 have shown upstream doesn't block Nightly
updates on errors in optional components. Do the same to avoid lagging
behind updates or the package disappearing due to undertesting.

src/bootstrap/dist.rs
1150: }).or_else(|| { missing_tool("RLS", builder.build.config.missing_tools); None })?;
1229: }).or_else(|| { missing_tool("clippy", builder.build.config.missing_tools); None })?;
1233: }).or_else(|| { missing_tool("cargo clippy", builder.build.config.missing_tools); None })?;
1310: }).or_else(|| { missing_tool("Rustfmt", builder.build.config.missing_tools); None })?;
1314: }).or_else(|| { missing_tool("Cargofmt", builder.build.config.missing_tools); None })?;