Update the install script for doppler to avoid apt-key

[MattHall]

I've updated the install script to avoid a call to apt-key as this was causing errors when building:

1.605 Reading package lists...
1.842 W: https://dl.yarnpkg.com/debian/dists/stable/InRelease: Policy will
reject signature within a year, see --audit for details
1.844 ./install.sh: 36: apt-key: not found
1.949 ERROR: Feature "doppler CLI"
(ghcr.io/metcalfc/devcontainer-features/doppler) failed to install!

The specs for atuin and dagger are both failing, but this isn't related to this PR as far as I can tell.

[metcalfc]

Thanks