appveyor: Cache meson's wrap downloads

This makes us less reliant on wrap-db (and reduces the amount of
downloading that goes on during the build).

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1936
Acked-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/appveyor.yml b/appveyor.yml
index 6c1bd37..3a24688 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -38,6 +38,7 @@
 - '%LOCALAPPDATA%\pip\Cache -> appveyor.yml'
 - win_flex_bison-2.5.15.zip
 - llvm-5.0.1-msvc2017-mtd.7z
+- subprojects\packagecache -> subprojects\*.wrap
 
 os: Visual Studio 2017