test: deflake env_install_interrupt by killing reinstall on install dir creation

crates/vite_cli_snapshots/tests/cli_snapshots/fixtures/env_install_interrupt/snapshots.toml

@@ -5,7 +5,7 @@ steps = [
   { argv = ["vp", "install", "-g", "./long-time-install-package"], continue-on-failure = true },
   { argv = ["long-time-install-package"], continue-on-failure = true },
   # snapshot = false: the killed install's partial screen is timing-dependent
-  # (which spinner frame was live at the 100ms SIGKILL) and ConPTY renders a
+  # (which spinner frame was live when the kill landed) and ConPTY renders a
   # trailing blank line Unix does not; the meaningful outcome is asserted by
   # the check-stale-packages step that follows.
   { argv = ["node", "test-reinstall-interrupt.js"], comment = "Reinstall but interrupt", snapshot = false, continue-on-failure = true },

crates/vite_cli_snapshots/tests/cli_snapshots/fixtures/env_install_interrupt/test-reinstall-interrupt.js

@@ -1,15 +1,42 @@
 const { spawn } = require('child_process');
+const fs = require('fs');
+const path = require('path');
+
+const scopeDir = path.join(process.env.VP_HOME, 'packages', '@scope');
+const before = new Set(fs.readdirSync(scopeDir));
 
 const child = spawn('vp', ['install', '-g', './long-time-install-package'], {
   stdio: 'inherit',
 });
 
-setTimeout(() => {
+// A fixed kill delay races vp's startup: on slow runners it can fire before
+// the reinstall creates its install dir, leaving nothing stale. Kill as soon
+// as the new dir appears; the package's 200ms postinstall keeps the install
+// running well past that point.
+const poll = setInterval(() => {
+  let entries;
+  try {
+    entries = fs.readdirSync(scopeDir);
+  } catch {
+    return;
+  }
+  if (entries.some((name) => !before.has(name))) {
+    clearInterval(poll);
+    child.kill('SIGKILL');
+  }
+}, 10);
+
+// Bound the wait so a reinstall that dies before creating the dir cannot
+// hang the case.
+const fallback = setTimeout(() => {
+  clearInterval(poll);
   if (!child.killed) {
     child.kill('SIGKILL');
   }
-}, 100);
+}, 10_000);
 
 child.on('close', (code) => {
+  clearInterval(poll);
+  clearTimeout(fallback);
   process.exit(code);
 });