_1password-cli: inline mainProgram in install and checks
- Replace `${pname}` reference in error message with explicit
`"1password-cli"` for clarity.
- Inline `op` instead of `${mainProgram}` in:
- `installPhase` binary installation
- shell completion installation
- `versionCheckProgram`
- Ensures consistency, avoids unnecessary indirection, and
makes the derivation easier to read and maintain.