Rollup merge of #143606 - lambdageek:configure-write-last-key, r=Kobzol

configure.py: Write last key in each section

The loop that writes the keys in each section of bootstrap.toml accumulates all the commented lines before a given key and emits them when it reaches the next key in the section.  This ends up dropping lines accumulated for the last key

Fixes rust-lang/rust#143605

Author: tgross35