installer.sh: accommodate linuxmint versioning.

Recent linuxmint releases now use VERSION_CODENAME for
a linuxmint release (like "uma") and set UBUNTU_CODENAME to
the Ubuntu release they branched from.

Tested in a linuxmint 20.2 VM.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>
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