Rollup merge of #64613 - alexcrichton:less-doc-copies, r=Mark-Simulacrum

rustbuild: Copy crate doc files fewer times

Previously when building documentation for the standard library we'd
copy all the files 5 times, and these files include libcore/libstd docs
which are huge! This commit instead only copies the files after rustdoc
has been run for each crate, reducing the number of redundant copies
we're making.