yes: increase 1st tee() size to 1 MiB

[oech3]

No reason to hesitate 1st write since zero-copy is extreamely fast.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/retry is no longer failing!
Congrats! The gnu test tests/timeout/timeout-group is no longer failing!
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.