Merge branch 'rimrul/prepare-for-ucrt64'

I took most of those changes, except the idea to build the MINGW64 and
UCRT64 packages in the same architecture: The long-term plan is to get
rid of this multi-arch building business, and instead build the packages
for a given `MINGW_ARCH` in their respective `git-sdk-*` (but for
historical reasons, I think we'll keep the names `git-sdk-32` for
mingw32, `git-sdk-64` for ucrt64 and `git-sdk-arm64` for clangarm64).

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

Author: dscho

.github/workflows/build-and-deploy.yml

@@ -256,10 +256,10 @@ jobs:
                     x86_64,*) echo "mingw64";;
                     i686,*) echo "mingw32";;
                     ucrt64,*) echo "ucrt64";;
-                    ,mingw-w64-wintoast) echo "mingw32 mingw64 clangarm64";; # We're (cross-)compiling via Visual Studio
-                    ,mingw-w64-git-credential-manager) echo "mingw32 mingw64 clangarm64";; # We're downloading the pre-built artifacts for all three platforms
-                    ,mingw-w64-git-lfs) echo "mingw32 mingw64 clangarm64";; # We're downloading the pre-built artifacts from Git LFS' official release page
-                    ,*) echo "mingw32 mingw64";;
+                    ,mingw-w64-wintoast) echo "mingw32 mingw64 ucrt64 clangarm64";; # We're (cross-)compiling via Visual Studio
+                    ,mingw-w64-git-credential-manager) echo "mingw32 mingw64 ucrt64 clangarm64";; # We're downloading the pre-built artifacts for all three platforms
+                    ,mingw-w64-git-lfs) echo "mingw32 mingw64 ucrt64 clangarm64";; # We're downloading the pre-built artifacts from Git LFS' official release page
+                    ,*) echo "mingw32 mingw64 ucrt64";;
                     esac
           )