ci-artifacts: build the minimal-sdk artifact for UCRT64, not MINGW64

The artifact's `/etc/profile` still hardcodes `MSYSTEM=MINGW64` and
`PATH=/mingw64/bin:...` (please.sh writes that on the build-extra side),
but on the ucrt64 branch the artifact actually ships `/ucrt64/bin/`.
Override `MSYSTEM` and prepend `/ucrt64/bin` to `PATH` after sourcing
`/etc/profile`, and update the gcc-path sanity check accordingly.

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

Author: dscho

.github/workflows/ci-artifacts.yml

@@ -51,8 +51,10 @@ jobs:
         run: |
           set -x
           . /etc/profile
+          export MSYSTEM=UCRT64
+          export PATH=/ucrt64/bin:$PATH
           test "$(cygpath -aw /)" = "${{github.workspace}}\minimal-sdk" || exit 1
-          test "$(type -p gcc)" = "/mingw64/bin/gcc" || exit 1
+          test "$(type -p gcc)" = "/ucrt64/bin/gcc" || exit 1
           make -C ../git DEVELOPER=1 NO_PERL=1 NO_RUST=UnfortunatelyYes SKIP_DASHED_BUILT_INS=YesPlease -j8 all strip
       - name: compress git artifacts
         shell: bash