Add a --disable-yjit job for Ubuntu CI

Currently, we do not have an amd64 job that runs with YJIT disabled
and runs the full check task.  Most amd64 jobs implicitly have YJIT
enabled, even though --enable-yjit was not specified.  This makes it
easier to determine whether a problem is YJIT-related or not by
reviewing CI logs.

.github/workflows/ubuntu.yml

@@ -36,6 +36,8 @@ jobs:
         include:
           - test_task: check
             arch: i686
+          - test_task: check
+            configure: '--disable-yjit'
           - test_task: check
             configure: '--enable-shared --enable-load-relative'
           - test_task: test-all TESTS=--repeat-count=2