Fix --from-pr feature for image load and stabilize help (#45575)

This is a follow-up after #45564 - it fixes the `--from-pr` and
`--from-run` to work (it was failing with file does not exist).

Also found out that gettempdir might return different directory
depending on which is your designated tmp directory (for example
in MacOS this is is a longer path in /var/.....) - so we have
to force the default during help generation to always return
"/tmp" so that the --help images do not change depending on which
system you are and what your tmp directory is.

Author: potiuk

dev/breeze/src/airflow_breeze/commands/ci_image_commands.py

@@ -679,9 +679,6 @@ def load(
     else:
         image_file_to_load = image_file_dir / image_file
 
-    if not image_file_to_load.exists():
-        get_console().print(f"[error]The image {image_file_to_load} does not exist.[/]")
-        sys.exit(1)
     if not image_file_to_load.name.endswith(f"-{python}.tar"):
         get_console().print(
             f"[error]The image file {image_file_to_load} does not end with '-{python}.tar'. Exiting.[/]"
@@ -698,6 +695,11 @@ def load(
         download_artifact_from_run_id(from_run, image_file_to_load, github_repository, github_token)
     elif from_pr:
         download_artifact_from_pr(from_pr, image_file_to_load, github_repository, github_token)
+
+    if not image_file_to_load.exists():
+        get_console().print(f"[error]The image {image_file_to_load} does not exist.[/]")
+        sys.exit(1)
+
     get_console().print(f"[info]Loading Python CI image from {image_file_to_load}[/]")
     result = run_command(["docker", "image", "load", "-i", image_file_to_load.as_posix()], check=False)
     if result.returncode != 0:

dev/breeze/src/airflow_breeze/commands/common_image_options.py

@@ -227,6 +227,6 @@
         readable=True,
         path_type=Path,
     ),
-    default=tempfile.gettempdir(),
+    default=tempfile.gettempdir() if not generating_command_images() else "/tmp",
     show_default=True,
 )