nix: ensure argv0 is correct when generating completions

nix/default.nix

@@ -24,9 +24,12 @@ buildFunction {
 
   nativeBuildInputs = [ installShellFiles ];
   postInstall = let
-  # "$out/bin/${mainProgram}" generate-completions --shell "${shell}" --command "${command}" > "$TMP/muscl.${shell}"
-    commands = lib.mapCartesianProduct ({ shell, command }: ''
-      COMPLETE=${shell} "$out/bin/${command}" > "$TMP/${command}.${shell}"
+    installShellCompletions = lib.mapCartesianProduct ({ shell, command }: ''
+      (
+        export PATH="$out/bin:$PATH"
+        export COMPLETE="${shell}"
+        "${command}" > "$TMP/${command}.${shell}"
+      )
       installShellCompletion "--${shell}" --cmd "${command}" "$TMP/${command}.${shell}"
     '') {
       shell = [ "bash" "zsh" "fish" ];
@@ -35,7 +38,9 @@ buildFunction {
   in ''
     ln -sr "$out/bin/muscl" "$out/bin/mysql-dbadm"
     ln -sr "$out/bin/muscl" "$out/bin/mysql-useradm"
-  '' + lib.concatStringsSep "\n" commands + ''
+
+    ${lib.concatStringsSep "\n" installShellCompletions}
+
     install -Dm444 assets/systemd/muscl.socket -t "$out/lib/systemd/system"
     install -Dm644 assets/systemd/muscl.service -t "$out/lib/systemd/system"
     substituteInPlace "$out/lib/systemd/system/muscl.service" \