check-for-missing-dlls: wire the workflow for the UCRT64 SDK

Three coupled changes are needed so that this workflow actually
exercises the new SDK rather than silently no-op'ing or pacman-erroring
against ghost MINGW64 packages.

  * Pass `--architecture=ucrt64` to `please.sh create-sdk-artifact`,
    so the resulting build-installers tree contains `/ucrt64/bin/`
    instead of an empty `/mingw64/bin/`.

  * Prepend `build-installers/ucrt64/bin` to `$GITHUB_PATH` (not
    `mingw64/bin`, which is now empty); without this, the dll-check
    step would find no binaries at all.

  * Set `MSYSTEM=UCRT64` for the two `check-for-missing-dlls.sh`
    invocations. The script branches on `$MSYSTEM` to pick its
    `MINGW_PREFIX` and the package set it asks pacman to install;
    without the env override it falls through to the *) case and
    tries to install the long-gone `mingw-w64-x86_64-*` packages.

All three depend on the supporting changes in
git-for-windows/build-extra#710.

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

Author: dscho

.github/workflows/check-for-missing-dlls.yml

@@ -33,13 +33,17 @@ jobs:
       - name: create build-installers artifact
         shell: bash
         run: |
-          sh -x ./build-extra/please.sh create-sdk-artifact --sdk=git-sdk-64.git build-installers &&
+          sh -x ./build-extra/please.sh create-sdk-artifact --architecture=ucrt64 --sdk=git-sdk-64.git build-installers &&
           cygpath -aw "$PWD/build-installers/usr/bin/core_perl" >>$GITHUB_PATH &&
           cygpath -aw "$PWD/build-installers/usr/bin" >>$GITHUB_PATH &&
-          cygpath -aw "$PWD/build-installers/mingw64/bin" >>$GITHUB_PATH
+          cygpath -aw "$PWD/build-installers/ucrt64/bin" >>$GITHUB_PATH
       - name: check for missing DLLs
         shell: bash
+        env:
+          MSYSTEM: UCRT64
         run: ./build-extra/check-for-missing-dlls.sh
       - name: check for missing DLLs (MinGit)
         shell: bash
+        env:
+          MSYSTEM: UCRT64
         run: MINIMAL_GIT=1 ./build-extra/check-for-missing-dlls.sh