Selecting earlier nodejs runtime version does not downgrade npm

[peter-at-work]

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 #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

[peter-at-work]

It looks like there are so many issues with npm v11 as found in https://github.com/npm/cli/issues about npm ci not being able to recognize lockfiles (that have not changed and previously worked with npm v10).

With projects still in nodejs v20, rolling back npm to v10 as a workaround will work, albeit clunky. But for those projects already in nodejs v24 which uses npm v11, they may not have a solution except to roll back npm to some earlier release of v11.

Now back here in AWS CodeBuild runner images, since nodejs v24 runtime is not yet officially listed as supported https://docs.aws.amazon.com/codebuild/latest/userguide/available-runtimes.html, and is still in a feature request, would rolling back the pre-installed npm to latest v10 automatically fix the issue, and gives AWS some time to figure out the npm v11 problem as part of offering nodejs v24 support?