Selecting earlier nodejs runtime version does not downgrade npm

**Describe the bug**

When an existing repo, developed with nodejs v20 (and npm v10), is npm-installed in CodeBuild via `npm ci`, an error is reported in CodeBuild:
```
`npm ci` can only install packages when your package.json and package-lock.json or npm-shrinkwrap.json are in sync. Please update your lock file with `npm install` before continuing.
```

**To Reproduce**
Steps to reproduce the behavior:
1. Use CodeBuild `aws/codebuild/standard:7.0` runner image
2. Use _runtime-versions_ of `nodejs: 20` in the buildspec file
3. Use a source repo that was created with nodejs v20 and npm v10, having a regular `package-lock.json` lockfile
4. Use command `npm ci` in the buildspec
5. The installed npm version (v11) fails in parsing/processing the `package-lock.json` lockfile

**Expected behavior**
`npm ci` should not throw an error.

**Platform (please complete the following information):**
- OS: Ubuntu; standard OS used in `aws/codebuild/standard:7.0`

**Additional context**
There seems to be a breaking change from npm v10 to npm v11. A recent PR https://github.com/aws/aws-codebuild-docker-images/pull/792 to the CodeBuild Dockerfile updated the pre-installed version of npm from v10 to v11. In CodeBuild runner images, selection of the nodejs runtime uses the `--preserve` flag or `N_PRESERVE_NPM` env variable which will keep the pre-installed npm version when switching nodejs runtime versions.

Default nodejs v20 and v22 installations use npm v10 https://nodejs.org/en/download/archive/v20.
Only nodejs v24 installations use npm v11 https://nodejs.org/en/download/archive/v24

In this case, nodejs runtime version is selected as v20, but the npm version remained at v11 which fails in processing the existing/unchanged `package-lock.json` lockfile.

**Potential workarounds**
- In a nodejs v20 development environment, upgrade npm from v10 to v11. Regenerate the `package-lock.json` lockfile by doing an `npm install`. Lockfile will be updated, and commit the changes to source code repo
- Add an `n --no-preserve 20` in the buildspec commands so that npm will be downgraded to v10 before running the `npm ci` command