oh-my-posh: add cache clearing on package version changes (#7757)

* add cache clearing for oh-my-posh package changes

Using oh-my-posh creates script files in `~/.cache/oh-my-posh` which include an old derivation path. Once that path is garbage collected, oh-my-posh stops working preventing to successfully create new shells.

```
bash: /nix/store/5ddhz8nsahf1d03smzx2xpmynjspjfh8-oh-my-posh-26.8.0/bin/oh-my-posh: No such file or directory
```

* fix oh-my-posh package reference in cache clear

Use the configured value from programs.oh-my-posh.package as the value of oh-my-posh to compare during cache cleanup.

modules/programs/oh-my-posh.nix

@@ -94,5 +94,21 @@ in
         }
       '';
     };
+
+    # Clear oh-my-posh cache when the oh-my-posh package derivation changes
+    home.activation.ohMyPoshClearCache = lib.hm.dag.entryAfter [ "writeBoundary" ] ''
+      set -eu
+      nixver="${config.programs.oh-my-posh.package}"
+      cache="${config.xdg.cacheHome}/oh-my-posh"
+      state="$cache/pkg-path"
+
+      if [ ! -f "$state" ] || [ "$(cat "$state")" != "$nixver" ]; then
+        rm -rf "$cache"
+      fi
+
+      mkdir -p "$cache"
+      printf '%s' "$nixver" > "$state"
+    '';
+
   };
 }