fix: Revert to readlink in generatePosixLink to avoid unnecessary unlinks

[polyomino24]

Context / Background

While running corepack enable without sudo in a CircleCI docker environment(cimg/node:22.20.0), the build failed with an EACCES: permission denied, unlink '/usr/local/bin/pnpm' error.

This error was caused by the recent change #761 from readlink to realpath in generatePosixLink.

https://github.com/nodejs/corepack/pull/761/files#diff-c91b034b85c9610409bf66de5d28560a06f1285ee1962f238fac2c3652a15e88L86-R88

Problem

Using fs.promises.realpath in generatePosixLink returns the fully resolved, absolute path of the symlink target.

However, the symlink variable used in the comparison is a relative path calculated by path.relative. This results in a comparison between an absolute path and a relative path, causing the currentSymlink !== symlink check to always evaluate to true.

corepack/sources/commands/Enable.ts

Lines 83 to 98 in cf0ddc1

	const symlink = path.relative(installDirectory, path.join(distFolder, `${binName}.js`));
	if (fs.existsSync(file)) {
	const currentSymlink = await fs.promises.realpath(file);
	if (binName.includes(`yarn`) && corepackUtils.isYarnSwitchPath(currentSymlink)) {
	console.warn(`${binName} is already installed in ${file} and points to a Yarn Switch install - skipping`);
	return;
	}
	if (currentSymlink !== symlink) {
	await fs.promises.unlink(file);
	} else {
	return;
	}
	}

[polyomino24]

@aduh95
Could you please re-run the failed tests?