fix: not allow install multiple package manager (pnpm#161)

When a project has both npm and pnpm, using pnpm/action-setup will result in an error: `Multiple versions of pnpm specified`.

The previous implementation was only meant to avoid the "ERR_PNPM_BAD_PM_VERSION" error, but it did not take into account the situation of multiple different package managers.

Signed-off-by: Kevin Cui <bh@bugs.cc>

Author: BlackHole1

src/install-pnpm/run.ts

@@ -64,6 +64,7 @@ async function readTarget(opts: {
   if (version) {
     if (
       typeof packageManager === 'string' &&
+      packageManager.startsWith('pnpm@') &&
       packageManager.replace('pnpm@', '') !== version
     ) {
       throw new Error(`Multiple versions of pnpm specified: