fix: Add fallback for npm exec path detection

When constants.npmExecPath from the published registry doesn't exist
or isn't executable, fall back to using whichBin to find npm. This
fixes CI failures where the published version's npm-exec-path module
might not correctly detect npm in certain environments.

Author: jdalton

src/utils/package-environment.mts

@@ -208,7 +208,13 @@ const LOCKS: Record<string, Agent> = {
 async function getAgentExecPath(agent: Agent): Promise<string> {
   const binName = binByAgent.get(agent)!
   if (binName === NPM) {
-    return constants.npmExecPath
+    // Try to use constants.npmExecPath first, but verify it exists.
+    const npmPath = constants.npmExecPath
+    if (existsSync(npmPath)) {
+      return npmPath
+    }
+    // If npmExecPath doesn't exist, fall back to whichBin.
+    return (await whichBin(binName, { nothrow: true })) ?? binName
   }
   return (await whichBin(binName, { nothrow: true })) ?? binName
 }