ci: Don't install Qt when running on self-hosted

The previous condition doesn't seem to work

.github/workflows/build.yml

@@ -67,13 +67,12 @@ jobs:
 
     steps:
       - name: Install Qt ${{ matrix.preset.qt_version }} with options and default aqtversion
-        if: env.IS_SELFHOSTED == null || env.IS_SELFHOSTED == ''
+        if: ${{ !defined(env.IS_SELFHOSTED) }}
         uses: jurplel/install-qt-action@v4
         with:
           version: ${{ matrix.preset.qt_version }}
           cache: true
 
-
       - name: Install dependencies on Ubuntu (${{ join(matrix.preset.apt_pgks, ' ') }})
         if: ${{ runner.os == 'Linux' && matrix.preset.apt_pgks }}
         run: |