Case-insensitive environment keys.
Pacify lint gods.
auto merge of #17249 : vadimcn/rust/env-keys, r=alexcrichton

Closes #16937
configure: Remove support for i686-pc-mingw32 triple
mk: Remove check-fast target
jemalloc: Un-revert accidentally reverted jemalloc
auto merge of #17330 : brson/rust/bot-house-cleaning, r=alexcrichton

Both these things were transitionary until bot upgrades.
Ignore two gdb tests on windows that are failing on the bots