Mount selected sources of Airflow to image when running mypy checks (#…

…36038)

When Mypy runs its check it uses breeze image to do so, and by
default on CI the images are built using `main` version of scripts/ci
and workflow files. This basically means that running static checks with
the breeze image reguies mounting the sources if we want to run those
checks on the scripts/ci/ folder or .workflow and breeze CI image is
used.

The `mypy-dev` check is doing it - it checks `scripts/ci` folder,
which means that for mypy checks we need to force mounting the
selected sources rather than the ones that are baked in the image.

scripts/ci/pre_commit/pre_commit_mypy.py

@@ -45,6 +45,10 @@
     warn_image_upgrade_needed=True,
     extra_env={
         "INCLUDE_MYPY_VOLUME": "true",
+        # Need to mount local sources when running it - to not have to rebuild the image
+        # and to let CI work on it when running on PRs from forks - because mypy-dev uses files
+        # that are not available at the time when image is built in CI
+        "MOUNT_SOURCES": "selected",
     },
 )
 if res.returncode != 0: