fetch-job.py: force non-buffered output

Before this change, all the output from fetch-job.py was printed after
all fetch job finished.

Change-Id: If413d1ee201b462d5035c30201436d5faecef195

container-images/fetch-job/Dockerfile

diff --git a/container-images/fetch-job/Dockerfile b/container-images/fetch-job/Dockerfile
index e0d0c30..b68b4e4 100644
--- a/container-images/fetch-job/Dockerfile
+++ b/container-images/fetch-job/Dockerfile
@@ -17,6 +17,8 @@
     python3 -m pip install --require-hashes -r requirements.txt --no-cache --upgrade && \
     python3 -m pipenv install --python 3.12 --system
 
+ENV PYTHONUNBUFFERED=1
+
 COPY tools/* /var/tools/
 
 USER gerrit