Verify checksum of plugin installation manager tool before executing

[basil]

docker-jenkins-lts/bin/update-plugins.sh

Line 15 in b40a31c

wget --no-verbose "${PM_CLI_DOWNLOAD_URL}" -O "${TMP_DIR}/jenkins-plugin-manager.jar"

downloads the plugin installation manager tool without verifying its checksum as in jenkinsci/docker#1868, meaning that a truncated download or corrupted file could result in an execution failure later on. Better to verify the checksum immediately and fail fast in that case.