ci: pin Qt install dir for act-runner (runner.temp comes through empty)

install-qt-action defaults `dir:` to `${{ runner.temp }}` if unset.
GitHub-hosted runners populate runner.temp; Gitea/Forgejo's act-runner
leaves it as the empty string. The action then rejects the empty path
with `TypeError: "dir" input may not be empty`.

Hardcoding `dir: ${{ github.workspace }}/qt` works on both — workspace
is always populated and writable, and the path is inside the job's
working tree so it's auto-cleaned with the workspace.

Same change in ci.yml and release.yml.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -55,6 +55,7 @@ jobs:
         with:
           version: '6.5.*'
           modules: 'qtquickcontrols2'
+          dir: ${{ github.workspace }}/qt
           cache: true
           setup-python: false