cmake, vcpkg: Pin libevent version

For more details, see bitcoin#30096.

Author: hebasto

vcpkg.json

@@ -2,6 +2,7 @@
   "$schema": "https://raw.githubusercontent.com/microsoft/vcpkg-tool/main/docs/vcpkg.schema.json",
   "builtin-baseline": "9edb1b8e590cc086563301d735cae4b6e732d2d2",
   "overrides":[
+    {"name": "libevent", "version": "2.1.12#7"},
     {"name": "liblzma", "version": "5.4.1#1"}
   ],
   "dependencies": [