nixos/virtualisation/podman: fix broken evaluation

- The following error occurs when building:

error: A definition for option `environment.systemPackages."[definition 8-entry 2]"' is not of type `package'. Definition values:
  - In `/nix/store/dgp9wmspgm6zhl66f0c81rvg3v7wwbsb-source/nixos/modules/virtualisation/podman/default.nix':
    ''
      /nix/store/yyspp8pf5fhpx17wb2ndrhdd7y5kqb4g-podman-docker-compat-5.7.0export HOME=$(mktemp -d) # work around `docker <cmd>`
        installShellCompletion --cmd docker \
          --bash <($out/bin/docker completion bash) \
          --zsh <($out/bin/docker completion zsh) \

- This error occurs because the string is being concatenated to the output
  of the function evaluation of `pkgs.runCommand` instead of being
  concatenated to the string input of the function.

- Tested locally and the build works again.

nixos/modules/virtualisation/podman/default.nix

@@ -24,23 +24,25 @@ let
         inherit (cfg.package) meta;
         preferLocalBuild = true;
       }
-      ''
-        mkdir -p $out/bin
-        ln -s ${cfg.package}/bin/podman $out/bin/docker
+      (
+        ''
+          mkdir -p $out/bin
+          ln -s ${cfg.package}/bin/podman $out/bin/docker
 
-        mkdir -p $man/share/man/man1
-        for f in ${cfg.package.man}/share/man/man1/*; do
-          basename=$(basename $f | sed s/podman/docker/g)
-          ln -s $f $man/share/man/man1/$basename
-        done
-      ''
-    + lib.optionalString (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
-      export HOME=$(mktemp -d) # work around `docker <cmd>`
-      installShellCompletion --cmd docker \
-        --bash <($out/bin/docker completion bash) \
-        --zsh <($out/bin/docker completion zsh) \
-        --fish <($out/bin/docker completion fish)
-    '';
+          mkdir -p $man/share/man/man1
+          for f in ${cfg.package.man}/share/man/man1/*; do
+            basename=$(basename $f | sed s/podman/docker/g)
+            ln -s $f $man/share/man/man1/$basename
+          done
+        ''
+        + lib.optionalString (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+          export HOME=$(mktemp -d) # work around `docker <cmd>`
+          installShellCompletion --cmd docker \
+            --bash <($out/bin/docker completion bash) \
+            --zsh <($out/bin/docker completion zsh) \
+            --fish <($out/bin/docker completion fish)
+        ''
+      );
 
 in
 {