Don't rely on the `npm_execpath` env variable to determine which package manager to use

[mikenikles]

The npm_execpath env variable isn't set if you install the package manager in a way other than npm i -g [yarn|pnpm].

This PR just checks if pnpm --version or yarn --version successfully runs. If so, it picks whichever command didn't fail. If both commands fail, we fall back to NPM.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: a14abd8ebee636b329008a0b8bb05f956a74737a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@webstone/cli	Minor
@webstone/core	Patch

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[mikenikles]

Julian, "This is only needed the very first time we create a Webstone app/plugin. Any other time, we can determine the package manager based on the available lock file."