ci: retire the busybox-w32 PR workaround for ash.exe

Until now, the workflow pulled `ash.exe` from an unmerged `busybox-w32` PR build because the `busybox.exe` shipped in the SDK was broken, and was separately also slow on account of being size-optimized. Both the correctness fix and a speed-optimized rebuild have since landed as a regular `mingw-w64-x86_64-busybox` package, so the in-flight workaround can go. I still assert a minimum installed version, because a stale SDK image could pin an older `busybox`, and I would much rather have CI fail loudly than silently run the test suite under a slow shell.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/main.yml

@@ -149,26 +149,17 @@ jobs:
       shell: bash
       run: tar xf artifacts.tar.gz && tar xf tracked.tar.gz
     - uses: git-for-windows/setup-git-for-windows-sdk@v2
-    - name: download BusyBox' ash.exe and use it as the POSIX shell
+    - name: install BusyBox' ash.exe and use it as the POSIX shell
       if: matrix.use-busybox == true
       shell: bash
-      env:
-        GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       run: |
-        # TEMPORARY: pull the freshly-built, heap-corruption-fixed ash.exe
-        # straight from the busybox-w32 PR build (git-for-windows/busybox-w32#15)
-        # instead of the stale copy shipped in git-sdk-64. Once that fix is
-        # released into the SDK this step reverts to the pinned download.
-        run_id="$(gh run list -R git-for-windows/busybox-w32 \
-          --branch fix-quote-arg-msys2-heap-corruption \
-          --workflow build.yml --status success --limit 1 \
-          --json databaseId --jq '.[0].databaseId')" &&
-        test -n "$run_id" &&
-        gh run download -R git-for-windows/busybox-w32 "$run_id" \
-          -n busybox -D bb &&
-        test -f bb/busybox.exe &&
-        cp bb/busybox.exe /mingw64/bin/ash.exe &&
-        ash="$(cygpath -am /mingw64/bin/ash.exe)" &&
+        pacman -Sy --noconfirm &&
+        pacman -S --noconfirm --needed mingw-w64-x86_64-busybox &&
+        installed="$(pacman -Q mingw-w64-x86_64-busybox | awk '{print $2}')" &&
+        echo "installed mingw-w64-x86_64-busybox: $installed" &&
+        test "$(vercmp "$installed" 1.38.0.21234.d8d8bb397-1)" -ge 0 ||
+          { echo "::error::busybox $installed is older than required 1.38.0.21234.d8d8bb397-1 (missing the speed-optimized build); see https://github.com/git-for-windows/busybox-w32/pull/16"; exit 1; } &&
+        ash="$(cygpath -am /mingw64/libexec/busybox/ash.exe)" &&
         echo "GIT_TEST_SHELL_PATH=$ash" >>$GITHUB_ENV &&
         echo "SHELL=$ash" >>$GITHUB_ENV &&
         echo "SHELL_PATH=$ash" >>$GITHUB_ENV &&