flock in compiling vendoring libraries failed for NFS

[SidongYang]

Hi,
Thanks for a great crate.
We're using this crate with vendored feature in NFS. But it failed to compile libelf.
It seems that flock in build.rs returns EBADF and it needs write permission in NFS.
I temporary fixed the code to give write permission in openning, I see it works.

https://man7.org/linux/man-pages/man2/flock.2.html