services.restic: use getExe instead of hard-coded path

Instead of assuming the restic executable is at `bin/restic`, use
`lib.getExe` on the restic package. The main motivation for this is
using a security wrapper that has a different name such as
`bin/restic-wrapper`.

NOTE: In rare cases this could be a breaking change. For example if you
are using a wrapper whose executable is at `bin/restic`,
`meta.mainProgram` is not set, _and_ your package name is something
other than `restic`.

See the [discource discussion](https://discourse.nixos.org/t/using-restic-service-with-the-security-wrapper/34547/5) for more details/discussion.

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