CI: improve gcov handling #10404

- Move __gcov_flush to process_spawn, for more reliable coverage
  tracking of subprocesses
- Travis: use GCOV_ERROR_FILE
- codecov: use "-X fix" to skip "fixing" uploaded coverage data; it
  should be handled by codecov's backend instead.
- AppVeyor: no $PATH mangling, which breaks with the improved coverage tracking
  due to missing .dll in PATH.

test/functional/eval/executable_spec.lua

@@ -21,8 +21,6 @@ describe('executable()', function()
     -- Windows: siblings are in Nvim's "pseudo-$PATH".
     local expected = iswin() and 1 or 0
     if iswin() then
-      -- $PATH on AppVeyor CI might be oversized, redefine it to a minimal one.
-      clear({env={PATH=[[C:\Windows\system32;C:\Windows]]}})
       eq('arg1=lemon;arg2=sky;arg3=tree;',
          call('system', sibling_exe..' lemon sky tree'))
     end