On Darwin, launchd may attempt to start agents before the Nix store is
mounted and available. This leads to failures when the agent's executable
or arguments reside in the Nix store.
This change wraps the agent's command in a shell script that uses
/bin/wait4path to ensure /nix/store is ready before executing the
original program. It also ensures that ProgramArguments are correctly
escaped and concatenated.
Alternative option for allowing a user to automatically configure what
binary to use from a `pinentry` package. Previously, we always used
`meta.mainProgram` but, there are packages that provide multiple
binaries and this would allow flexibility for a user to override the
default program used.
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
