Try --x-use-aria2 for better retries (#6574)

to possibly deal with failures like https://ci.appveyor.com/project/ruby/ruby/builds/45097615.

I'm not sure how vcpkg retries things, but at least aria2c seems to attempt downloads 5 times by default.
https://aria2.github.io/manual/en/html/aria2c.html

Looking at the CI log, vcpkg might be already trying it 5 times, but let me give it a try too.

Author: k0kubun

.appveyor.yml

@@ -46,7 +46,7 @@ for:
     - git pull -q
     - .\bootstrap-vcpkg.bat
     - cd %APPVEYOR_BUILD_FOLDER%
-    - vcpkg --triplet %Platform%-windows install libffi libyaml readline zlib
+    - vcpkg --triplet %Platform%-windows install --x-use-aria2 libffi libyaml readline zlib
     - CALL SET vcvars=%%^VS%VS%COMNTOOLS^%%..\..\VC\vcvarsall.bat
     - SET vcvars
     - '"%vcvars%" %Platform:x64=amd64%'