Remove corepack upgrade step from corepack build target

The corepack step as it exists causes issues if a machine already has corepack enabled and is using the npm shim:
```
olszewski@chriss-mbp cli % make corepack
npm install -g corepack@latest
Usage Error: This project is configured to use pnpm

$ npm ...
make: *** [corepack] Error 1
```

I think we could get around this by changing to a working directory without a packageManager, but that feels very icky.  Since we were only upgrading corepack in order to avoid [#110](nodejs/corepack#110) which only happens the first time a user sets up a package manager on a machine, I think this an acceptable regression in order to unblock development.

We should follow what comes out of [vercel#157](nodejs/corepack#157) to see if we can add this back eventually.

cli/Makefile

@@ -67,7 +67,7 @@ install: | ./package.json
 	pnpm install --filter=cli
 
 corepack:
-	npm install -g corepack@latest
+	which corepack || npm install -g corepack@latest
 	corepack enable
 
 e2e: corepack install turbo