auto merge of #20437 : ranma42/rust/fix-make-install, r=alexcrichton

After 8b3c67690c4747b9fadfef407e6261524fb03f8a the `make install`
command fails if docs are not disabled through CFG_DISABLE_DOCS,
because now the `install` target uses
../../tmp/dist/$(DOC_PKG_NAME)-$(CFG_BUILD)/install.sh

Instead of explicitly depending on
dist/$(PKG_NAME)-$(CFG_BUILD).tar.gz, the `prepare_[un]install`
targets now depend on `dist-tar-bins`, which packages the appropriate
dist archives depending on the configuration.