Installer of 20210225 snapshot works with monochrome mode

[Yasuhiro Kimura]

I created VirtualBox VM on Windows host and tried to make clean
install of 14-CURRENT with iso image of 20210225 snapshot
(FreeBSD-14.0-CURRENT-amd64-20210225-bf667f282a7-256946-disc1.iso). Then
installer worked with monochrome mode as following.

https://www.utahime.org/FreeBSD/install-snapshot-020210225.01.png
https://www.utahime.org/FreeBSD/install-snapshot-020210225.02.png
https://www.utahime.org/FreeBSD/install-snapshot-020210225.03.png

If I start `bsdinstall` after installation is completed, then it works
normal color mode. So it seems bug of install image.

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Yasuhiro Kimura