Rollup merge of #136815 - ChrisDenton:fix-mingw-ci, r=Kobzol

CI: Stop /msys64/bin from being prepended to PATH in msys2 shell

We used to do this along time ago but we stopped doing it when we started installing msys2 manually. https://github.com/rust-lang/rust/blob/4fd3cf96a1db7771ef4f332b9eb1ad17fa0fd091/src/ci/scripts/install-msys2.sh#L11-L13

Fixes #136795

try-job: dist-i686-mingw

Author: matthiaskrgr