azure: Switch to use the MSYS2 official installer as the CI base

Recent CI failures were seen [1] when building MSYS2 Windows host
tools. The error messages are something like:

  downloading mingw32.db...
  downloading mingw32.db.sig...
  error: mingw32: key "4A6129F4E4B84AE46ED7F635628F528CF3053E04" is unknown
  error: mingw32: signature from "David Macek <david.macek.0@gmail.com>" is unknown trust
  error: failed to update mingw32 (invalid or corrupted database (PGP signature))

Per the MSYS2 official news [2], this was caused by a packager
switch and several solutions were suggested, e.g.: a new package
of msys2-keyring and a new msys2 installer that includes them are
released. However right now we have been using the MSYS2 github
CI base repo [3] for the MSYS2 build in U-Boot, but per the project
information on the github webpage, it says: "This repository is
unused/deprecated and will be remove after 2021-01-01". Since it is
unmaintained it's unlikely the new PGP keys will be included in the
git repo, and the only choice is to switch to use the MSYS2 official
installer as the CI base instead.

[1] https://dev.azure.com/u-boot/u-boot/_build/results?buildId=975
[2] https://www.msys2.org/news/#2020-06-29-new-packagers
[3] https://github.com/msys2/msys2-ci-baseSigned-off-by: NBin Meng <bmeng.cn@gmail.com>

.azure-pipelines.yml

@@ -15,10 +15,11 @@ jobs:
     pool:
       vmImage: $(windows_vm)
     steps:
-      - script: |
-          git clone https://github.com/msys2/msys2-ci-base.git %CD:~0,2%\msys64
+      - powershell: |
+          (New-Object Net.WebClient).DownloadFile("https://github.com/msys2/msys2-installer/releases/download/2020-07-20/msys2-base-x86_64-20200720.sfx.exe", "sfx.exe")
         displayName: 'Install MSYS2'
       - script: |
+          sfx.exe -y -o%CD:~0,2%\
           %CD:~0,2%\msys64\usr\bin\bash -lc "pacman --noconfirm -Syyuu"
         displayName: 'Update MSYS2'
       - script: |