Fix multiline run_install example in README.md (#167)

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>

README.md

@@ -117,7 +117,7 @@ jobs:
       - uses: pnpm/action-setup@v4
         with:
           version: 10
-          run_install: |
+          run_install:
             - recursive: true
               args: [--frozen-lockfile, --strict-peer-dependencies]
             - args: [--global, gulp, prettier, typescript]

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: