try CW_LLVM_MINGW_ONLY=1

It reduces the extra installed packages to half. The remaining ones
belonging to `wine` mostly.

Since llvm-mingw is used for ARM64 anyway, this is a pure saving.
Downside is shifting away from the standard distro track, where Debian
Trixie ships with mingw-w64 12, while debian:testing has 13 now and
llvm-mingw has 14 (staying close to master), and they will be bumped
further away from Trixie and other Linux distros in the coming years.
In theory these should be compatible. In practice, not always, also
depending on what we use exactly.

Ref: 21cdc44

These builds are at least by trurl CMake tests on Windows:
https://github.com/curl/trurl/blob/a96ea111f206c72462c05ae3ecc494c0875b14b8/.github/workflows/build.yml#L197-L204

win-llvm:

Before:
```
0 upgraded, 288 newly installed, 0 to remove and 10 not upgraded.
Need to get 413 MB of archives.
After this operation, 2185 MB of additional disk space will be used.
```
https://github.com/curl/curl-for-win/actions/runs/18853837259/job/53796629205

After:
```
0 upgraded, 237 newly installed, 0 to remove and 10 not upgraded.
Need to get 237 MB of archives.
After this operation, 1152 MB of additional disk space will be used.
```
https://github.com/curl/curl-for-win/actions/runs/18856581133/job/53805849066

Author: vszakats

.github/workflows/build.yml

@@ -609,7 +609,7 @@ jobs:
           COSIGN_PKG_GPG_PASS: '${{ secrets.COSIGN_PKG_GPG_PASS }}'
           COSIGN_PKG_KEY_PASS: '${{ secrets.COSIGN_PKG_KEY_PASS }}'
           CW_LLVM_MINGW_DL: '1'
-          CW_LLVM_MINGW_ONLY: '0'
+          CW_LLVM_MINGW_ONLY: '1'
         run: |
           export CW_CONFIG="${GITHUB_REF_NAME}-werror-win"
           export CW_REVISION="${GITHUB_SHA}"
@@ -640,7 +640,7 @@ jobs:
           COSIGN_PKG_GPG_PASS: '${{ secrets.COSIGN_PKG_GPG_PASS }}'
           COSIGN_PKG_KEY_PASS: '${{ secrets.COSIGN_PKG_KEY_PASS }}'
           CW_LLVM_MINGW_DL: '1'
-          CW_LLVM_MINGW_ONLY: '0'
+          CW_LLVM_MINGW_ONLY: '1'
         run: |
           export CW_CONFIG="${GITHUB_REF_NAME}-werror-win"
           export CW_REVISION="${GITHUB_SHA}"