The link to the prod.list file in tools/installpsh-debian.sh is wrong:
https://github.com/PowerShell/PowerShell/blob/master/tools/installpsh-debian.sh#L129

It should not be

curl https://packages.microsoft.com/config/ubuntu/$REV/prod.list | $SUDO tee /etc/apt/sources.list.d/microsoft.list

but

curl https://packages.microsoft.com/config/debian/8/prod.list | $SUDO tee /etc/apt/sources.list.d/microsoft.list

Indeed if you go to https://packages.microsoft.com/config/debian, you can see that there is only one directory (8), so it's not even needed to use the REV variable, which moreover is set to "8.9" on my machine.