matrix: add el8 tag and make it test-install

problem: we're not hooking up to the el8 container from core properly

solution: tag the container after the test, and match the core container
with test-install

src/test/generate-matrix.py

@@ -268,10 +268,12 @@ def __str__(self):
     name="el8",
     image="el8",
     env=dict(
+        TEST_INSTALL="t",
         # this is _required_ because of valgrind's new dependency on python3.11
         # which confuses rhel8 cmake's detection logic
-        PYTHON="/usr/bin/python3.6"
+        PYTHON="/usr/bin/python3.6",
     ),
+    docker_tag=True,
 )
 
 print(matrix)