fix: Install asdf plugins before updating variables

"asdf current" reports installed versions, not just what's in .tool-versions, so if we don't run ./install-asdf-plugins.sh first then running the update command will commonly remove some or all of the entries in .gitlab-ci-asdf-versions.yml, depending on what asdf plugins are installed in the image running this update.

An example of this occuring is https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/sandbox/switchboard_la/-/merge_requests/36/diffs?commit_id=70f712ad9b4528af356f02b8e81fc64fe4b5ac70

renovate-common.json

@@ -85,7 +85,7 @@
       "extractVersion": "^v?(?<version>.*)",
       "postUpgradeTasks": {
         "commands": [
-          "if [ -x ./scripts/update-asdf-version-variables.sh ]; then ASDF_DIR=/asdf ./scripts/update-asdf-version-variables.sh; fi"
+          "if [ -x ./scripts/update-asdf-version-variables.sh ]; then ASDF_DIR=/asdf ./scripts/install-asdf-plugins.sh && ./scripts/update-asdf-version-variables.sh; fi"
         ],
         "fileFilters": [
           ".gitlab-ci-*.yml"