_1password-gui: inline pname in desktop file installation

- Replace `${pname}` with explicit `1password` in desktop file
installation and substitution.
- Ensures consistency with upstream naming and avoids unnecessary
indirection in paths and Exec entries.

pkgs/by-name/_1/_1password-gui/linux.nix

···
       cp -a * $out/share/1password
 
       # Desktop file
-      install -Dt $out/share/applications resources/${pname}.desktop
-      substituteInPlace $out/share/applications/${pname}.desktop \
-        --replace-fail 'Exec=/opt/1Password/${pname}' 'Exec=${pname}'
+      install -Dt $out/share/applications resources/1password.desktop
+      substituteInPlace $out/share/applications/1password.desktop \
+        --replace-fail 'Exec=/opt/1Password/1password' 'Exec=1password'
 
     ''
     + (lib.optionalString (polkitPolicyOwners != [ ]) ''