Update Corepack instructions in installation.md

[aduh95]

The current instructions were a bit outdated, so I'm suggesting a few updates:

• corepack enable currently also installs Yarn. It might make sense to restrict it to pnpm.
• Since feat: download the latest version instead of a pinned one nodejs/corepack#134, the version folks get when installing Corepack should always be the latest.
• We could talk about package-pinning with corepack use.

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[zkochan]

what if I don't want to use the packageManager field just install the latest pnpm version globally?

[aduh95]

You should expect Corepack to download the latest version on the first use. After that, if you want to manually bump it, you can run corepack install --global pnpm@latest (or the old corepack prepare command, that still works).

[zkochan]

So corepack enable pnpm installs the latest version? It used to install some hardcoded stable version.

[aduh95]

The short answer is "yes". The long answer is:
Technically, corepack enable pnpm only adds a binary on the PATH; pnpm will be downloaded only when it's first needed (so either when calling corepack pnpm … or using the pnpm bin created by corepack enable), and here are the steps that Corepack takes to determine what version to use:

• if there's a local package.json with a "packageManager" field, use the version defined there.
• otherwise, check if there's a cached global version, if so, use that.
• otherwise, check if env variable COREPACK_DEFAULT_TO_LATEST was set to 0, in which case, use the hard coded version.
• otherwise, use the latest version from the npm registry.
  It did use to default to the hardcoded version, but that was a while back.