Update how Mockery mocks are checked for currency. (#8697)

The use of "go install" is deprecated as a way of installing and running the
Mockery binary. Update the runner script to depend on an ambient version of the
tool and ensure that in CI it's installed.

.github/workflows/check-generated.yml

@@ -25,6 +25,9 @@ jobs:
       - name: "Check generated mocks"
         run: |
           set -euo pipefail
+
+          readonly MOCKERY=2.12.3  # N.B. no leading "v"
+          curl -sL "https://github.com/vektra/mockery/releases/download/v${MOCKERY}/mockery_${MOCKERY}_Linux_x86_64.tar.gz" | tar -C /usr/local/bin -xzf -
           make mockery 2>/dev/null
 
           if ! git diff --stat --exit-code ; then