CI/AppVeyor: Disable gcov build for PRs

Currently the "gcov" build always fails on AppVeyor. It makes the builds
very slow, so disable it for PRs until the problem is fixed.

closes #8911
closes #8912

ci/build.ps1

@@ -14,7 +14,7 @@ $nvimCmakeVars = @{
 }
 
 # For pull requests, skip some build configurations to save time.
-if ($env:APPVEYOR_PULL_REQUEST_HEAD_COMMIT -and $env:CONFIGURATION -match '^(MSVC_64|MINGW_32)$') {
+if ($env:APPVEYOR_PULL_REQUEST_HEAD_COMMIT -and $env:CONFIGURATION -match '^(MSVC_64|MINGW_32|MINGW_64-gcov)$') {
   exit 0
 }