appveyor: add allow_failures for vs2017 and zstd

The input line is too long.
The syntax of the command is incorrect.
Command exited with code 255
https://ci.appveyor.com/project/myfreeer/7z-build-nsis/builds/49827526/job/li0tbw56c27tje9w#L2786

Author: myfreeer

appveyor.yml

@@ -1,4 +1,4 @@
-version: 21.03-{build}
+version: 24.05-{build}
 image:
 - Visual Studio 2022
 - Visual Studio 2017
@@ -19,3 +19,8 @@ skip_commits:
     - 'LICENSE'
     - '*.md'
     - '.gitingore'
+
+matrix:
+  allow_failures:
+    - image: Visual Studio 2017
+      src: zstd