Auto merge of #136815 - ChrisDenton:fix-mingw-ci, r=<try>

Prevent Git4Win from prepending its bin directory to `PATH`

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: bors

Diff for: src/ci/scripts/install-mingw.sh

@@ -32,6 +32,11 @@ if isWindows && isKnownToBeMingwBuild; then
             ;;
     esac
 
+    # Stop Git4Win from prepending /msys64/bin to PATH.
+    msys2Path="c:/msys64"
+    mkdir -p "${msys2Path}/home/${USERNAME}"
+    ciCommandAddPath "${msys2Path}/usr/bin"
+
     mingw_dir="mingw${bits}"
 
     curl -o mingw.7z "${MIRRORS_BASE}/${mingw_archive}"