fix(envd): return error when concurrent /init races on NFS mount

[AdaAibaby]

When two concurrent /init requests race on isMountingNFS, the CompareAndSwap guard in setupNFS returns the named error value e, which is nil. The second caller proceeds as if NFS mounted successfully — sandbox starts with no volume mounts and no error surfaced to the caller.

Fix: return a package-level sentinel ErrConcurrentNFSInit so the caller receives a 500 and can retry. Using a sentinel (rather than fmt.Errorf) allows callers to match the error with errors.Is(), consistent with ErrAccessTokenMismatch and other sentinels in the same file.

Change

// before
if !a.isMountingNFS.CompareAndSwap(false, true) {
    logger.Debug().Msg("NFS volumes already mounting")
    return e  // nil — silent failure
}

// after
var ErrConcurrentNFSInit = errors.New("NFS mount already in progress")

if !a.isMountingNFS.CompareAndSwap(false, true) {
    logger.Debug().Msg("NFS volumes already mounting")
    return ErrConcurrentNFSInit
}

1 file, 2 lines changed.

/cc @jakubno @dobrac @ValentaTomas @arkamar @tvi Looking forward to your code review.