Stop reinstalling identical packages in our Windows CI

Fixes bug 27464; bugfix on 0.3.4.1-alpha.

.appveyor.yml

@@ -26,7 +26,7 @@ install:
     {
         Execute-Command 'c:\msys64\usr\bin\bash' '-e' '-c' $args
     }
-    Execute-Command "C:\msys64\usr\bin\pacman" -Sy --noconfirm openssl-devel openssl libevent-devel libevent mingw-w64-i686-libevent mingw-w64-x86_64-libevent mingw-w64-i686-openssl mingw-w64-x86_64-openssl mingw-w64-i686-zstd mingw-w64-x86_64-zstd
+    Execute-Command "C:\msys64\usr\bin\pacman" -Sy --needed --noconfirm openssl-devel openssl libevent-devel libevent mingw-w64-i686-libevent mingw-w64-x86_64-libevent mingw-w64-i686-openssl mingw-w64-x86_64-openssl mingw-w64-i686-zstd mingw-w64-x86_64-zstd
 
 build_script:
 - ps: >-

changes/bug27464

@@ -0,0 +1,3 @@
+  o Minor bugfixes (continuous integration):
+    - Stop reinstalling identical packages in our Windows CI.
+      Fixes bug 27464; bugfix on 0.3.4.1-alpha.