timescaledb: create $out/bin in order to work around a stupid bug

See #22653 or #38469. 'manveru' reported this on IRC again, but unfortunately a
real fix requires a bit more work. In the mean time, this plugs the leak with a
bit of duct tape.

'timescale' is an often desired extension and, in the mean time, this
workaround means you also do not have to install something like postgis (with a
large dependency set) into your closure to get things to work.

Ideally in the mean time, *all* postgresql extensions should have $out/bin
created for them... Doing this will require more ongoing work, or a proper
fix to the buildEnv nonsense going on.

Signed-off-by: Austin Seipp <aseipp@pobox.com>