AppVeyor: only save cache on master

.appveyor.yml

@@ -8,7 +8,7 @@ environment:
   GIT_LFS_SKIP_SMUDGE: 1
 
 cache:
-  - NimBinaries
+  - NimBinaries -> .appveyor.yml
 
 matrix:
   # We always want 32-bit and 64-bit compilation
@@ -21,6 +21,10 @@ platform:
 # when multiple CI builds are queued, the tested commit needs to be in the last X commits cloned with "--depth X"
 clone_depth: 10
 
+init:
+  # disable cache saving outside the master branch
+  - ps: IF ("$env:APPVEYOR_REPO_BRANCH" -ne "master") { $env:APPVEYOR_CACHE_SKIP_SAVE = "true" }
+
 install:
   - git submodule update --init --recursive
   # use the newest versions documented here: https://www.appveyor.com/docs/windows-images-software/#mingw-msys-cygwin