Fix include style in exec/sequence.hpp

The header `exec/sequence.hpp` has always used the `< >` style for includes,
e.g. `#include <stdexec/execution.hpp>`.  This doesn't work with NVC++.
But that hasn't been a problem in practice because none of the examples
or tests seem to have included `exec/sequence.hpp`.  But a few days ago
`exec/repeat_n.hpp` and `exec/repeat_effect_until.hpp` were both changed
to have `#include "sequence.hpp"`.  That broke many of the NVC++ tests
for stdexec with
```
".../sequence.hpp", line 18: catastrophic error: cannot open source file "stdexec/execution.hpp"
  #include <stdexec/execution.hpp>
                                  ^
```

Fix the failures by changing `exec/sequence.hpp` to use relative
includes in quotes rather than `< >`.  This fixes the NVC++ tests.

Author: dkolsen-pgi

include/exec/sequence.hpp

@@ -15,9 +15,9 @@
  */
 #pragma once
 
-#include <stdexec/execution.hpp>
-#include <stdexec/__detail/__tuple.hpp>
-#include <stdexec/__detail/__variant.hpp>
+#include "../stdexec/execution.hpp"
+#include "../stdexec/__detail/__tuple.hpp"
+#include "../stdexec/__detail/__variant.hpp"
 
 STDEXEC_PRAGMA_PUSH()
 STDEXEC_PRAGMA_IGNORE_GNU("-Wmissing-braces")