Allow pnpm to install packages

[jimohalloran]

In the transition from npm to pnpm the changeover from running npm ci to pnpm i --lockfile-only resulted in incorrect behaviour.

The npm ci command downloaded and installed packages at the specific versions as specified in the lockfile.

pnpm i --lockfile-only on the other hand does not download dependencies, it just updates the lockfile. This is not a substitute for npm ci.

What we want instead of pnpm i --frozen-lockfile as this will download and install exactly the dependencies specified in the lock file, which is equivelent to the npm ci command.

The problem with using --lockfile-only is that when trying to deploy outside of dev or CI environments (--frozen-lockfile is by default on in CI) no npm dependencies are downloaded or installed, which results in a broken system.

This patch updates the command line used by the composer helper script accordingly.

[mbessolov]

@jimohalloran Thank you for your contribution!
The proposed changes will be merged and included in an upcoming release.

[mbessolov]

internal ticket - BB-27327