ci: AppVeyor: fix cov job, remove duplicate non-cov one (#10217)

* ci: AppVeyor: set GCOV_ERROR_FILE

This prevents the warnings/errors to be spilled into test results,
causing them to fail them, e.g.:

    [  FAILED  ] C:/projects/neovim/test/functional\core\main_spec.lua @ 97: Command-line option -s errors out when trying to use nonexistent file with -s
    C:/projects/neovim/test/functional\core\main_spec.lua:98: Expected objects to be the same.
    Passed in:
    (string) 'Cannot open for reading: "Xtest-functional-core-main-s.nonexistent": no such file or directory
    profiling:C:\projects\neovim\build/src/nvim/CMakeFiles/nvim.dir/buffer.c.gcda:Data file mismatch - some data files may have been concurrently updated without locking support
    '
    Expected:
    (string) 'Cannot open for reading: "Xtest-functional-core-main-s.nonexistent": no such file or directory
    '

    stack traceback:
            C:/projects/neovim/test/functional\core\main_spec.lua:98: in function <C:/projects/neovim/test/functional\core\main_spec.lua:97>

For reference, the locking appears to have been reworked for gcc 9.1 [1].

1: https://github.com/gcc-mirror/gcc/commit/56621355b

helpers.clear: keep GCOV_ERROR_FILE in environment

* ci: AppVeyor: remove MINGW_64 config (used with cov now)

Also:

- run MINGW_64-gcov first, and with PRs, since it provides coverage.

test/functional/helpers.lua

@@ -430,6 +430,7 @@ local function clear(...)
         'PATH',
         'NVIM_LOG_FILE',
         'NVIM_RPLUGIN_MANIFEST',
+        'GCOV_ERROR_FILE',
       }) do
         if not env_tbl[k] then
           env_tbl[k] = os.getenv(k)