scripts/installer: support LinuxMint Debian.

The primary distribution for LinuxMint is based on Ubuntu,
but there is an alternate Debian-based distribution called
LMDE. Both variations identify themselves as "linuxmint"

We added UBUNTU_VERSION to the Ubuntu handling for linuxmint,
the only distribution so far found to do this. Instead, split
linuxmint out into its own case and use either UBUNTU_VERSION
or DEBIAN_VERSION, whichever is present.

Tested on an LMDE 5 (elsie) VM.

Updates https://github.com/tailscale/tailscale/issues/2915

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