gwms-factory: Move el7 pyyaml install to its own RUN section

If the yum command fails, it leads to a confusing pip3 attempt on EL9

opensciencegrid/gwms-factory/Dockerfile

@@ -13,16 +13,18 @@ ARG BASE_OS
 # the web-area
 RUN groupadd -o -g 996 gfactory && useradd -o -u 996 -g 996 -s /bin/sh gfactory
 
-# Note: EL7 and EL8 PyYAML RPM version is too old to include yaml.FullLoader (v5.1+)
-#       for glideinwms/lib/config_util.py
 RUN    yum -y install \
          git \
          glideinwms-factory \
          msmtp \
          vo-client \
          vim-enhanced \
-    && [[ ${BASE_OS} == 'el9' ]] \
-       || pip3 install --no-cache-dir PyYAML \
     && rm -rf /var/cache/yum/*
 
+# Note: EL7 and EL8 PyYAML RPM version is too old to include yaml.FullLoader (v5.1+)
+#       for glideinwms/lib/config_util.py
+RUN if [[ ${BASE_OS} < 'el9' ]] ; then \
+      pip3 install --upgrade --no-cache-dir PyYAML \
+    ; fi
+
 ADD etc /etc