Revert "the conditional did not work"

This reverts commit 7249f9b.

Author: awaelchli

.github/workflows/ci-pkg-install.yml

@@ -61,11 +61,9 @@ jobs:
         if: ${{ matrix.pkg-name == 'lightning' }}
         run: python -m lightning --version
 
-      - name: Ignore import mismatch on Python 3.7
-        if: ${{ matrix.python-version == '3.7' }}
-        run: export PY_IGNORE_IMPORTMISMATCH=1
-
       - name: DocTest package
+        env:
+          PY_IGNORE_IMPORTMISMATCH: ${{ matrix.python-version == '3.7' }}
         run: |
           PKG_NAME=$(python -c "print({'app': 'lightning_app', 'lite': 'lightning_lite', 'pytorch': 'pytorch_lightning', 'lightning': 'lightning'}['${{matrix.pkg-name}}'])")
           python -m pytest src/${PKG_NAME} --ignore-glob="**/cli/*-template/**"