ci: install tools for renovate post-upgrade tasks (#38815)

Renovate installs a toolchain only for the managers a branch actually
updates. A branch without a gomod update therefore has no `go` on
`PATH`, and `make tidy`
[fails](https://github.com/go-gitea/gitea/pull/38813#issuecomment-5211013198).

Fix by installing those tools. `node` is not needed because it comes
with renovate.

---------

Signed-off-by: silverwind <me@silverwind.io>

renovate.json5

@@ -19,6 +19,10 @@
   },
   "postUpgradeTasks": {
     "commands": ["make tidy", "make svg", "make generate-codemirror-languages"],
+    "installTools": {
+      "golang": {},
+      "pnpm": {},
+    },
     "fileFilters": [
       "go.mod",
       "go.sum",