fix(path): exepath() should respect 'shellslash' #39541

Problem:
`gx` relies on `exepath` to get the fullpath of `cmd.exe`,
and that path must use `\`; otherwise, luv's spawn will fail.

Solution:
Revert `slash_adjust` in `exepath`, so that it still respects 'shellslash'

test/functional/vimscript/exepath_spec.lua

@@ -76,4 +76,10 @@ describe('exepath()', function()
       end
     )
   end
+
+  it('respects shellslash #39524', function()
+    t.skip(not is_os('win'), "N/A: 'shellslash' only works on Windows")
+    local path = call('exepath', 'cmd.exe') ---@type string
+    t.ok(nil == path:find('/'))
+  end)
 end)