Add vital missing step to MariaDB 10.5 upgrade job to actually install
the new binary being built. Without this the test was happily passing all
the time but actually not testing the upgrade.

Also stop using oneliner syntax for the install step to make the debugging
of failing installs/upgrades from build logs easier.

NOTE TO MERGERS: This commit is made on 10.6 branch and can be merged to
all later branches (10.7, 10.8, ..., 11.0). If/when some jobs break, they
will be fixed per branch on follow-up commits.