Add lock file to prevent concurrent `func update` invocations

### Description

`func update` has no interprocess protection against concurrent invocations on the same machine. Two simultaneous updates could race on renaming and copying the same files in the install directory, leading to a corrupted installation.

Add a lock file (e.g. `func.update.lock` in the install directory) that is acquired at the start of `UpdateAsync` and released on completion. If the lock is already held, exit early with a user-friendly message.

Start at the UpdateAsync entry point and trace how the install directory is selected and how completion or early failure is handled. Add protection using a lock file in that directory, release it when the update finishes, and verify that a concurrent invocation exits with a user-friendly message without corrupting the installation.

Written by the indexing model from the issue text.