Cache more of rust. (#6309)
Cache more of rust. (#6309)

Picks the lowest hanging fruit from #6119.

twitter/pants,tdyas/pants,wisechengyi/pants,pantsbuild/pants,twitter/pants,wisechengyi/pants,tdyas/pants,tdyas/pants,twitter/pants,wisechengyi/pants,wisechengyi/pants,jsirois/pants,benjyw/pants,tdyas/pants,pantsbuild/pants,jsirois/pants,wisechengyi/pants,benjyw/pants,twitter/pants,pantsbuild/pants,pantsbuild/pants,tdyas/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,twitter/pants,pantsbuild/pants,twitter/pants,tdyas/pants,wisechengyi/pants,jsirois/pants,benjyw/pants,wisechengyi/pants,twitter/pants,tdyas/pants,tdyas/pants,twitter/pants,benjyw/pants,benjyw/pants,wisechengyi/pants,twitter/pants,wisechengyi/pants,pantsbuild/pants,tdyas/pants