Procedural language extensions can't be enabled locally

[olirice]

When running the test suite via

nix flake check -L

Some extensions fail to enable.

plv8
plcoffee
plls

all throw the error could not load library... symbol not found in flat namespace

To reproduce

nix run .#start-server 15

in another terminal

nix run .#start-client 15

and finally

create extension plv8;

[samrose]

PR to close this issue #1117