Patch to support nix 0.23 #17

In Debian we try where possible to keep only one version of each crate in the archive and we bump dependenices and apply patches accordingly. While testing your crate with a new version of nix I discovered that a slight code change was needed as "as_errno" was deprecated and made into a no-op in nix 0.22 and removed in nix 0.26.

Our patch can be found at https://salsa.debian.org/rust-team/debcargo-conf/-/blob/master/src/libmount/debian/patches/nix.diff

Note: in Debian we set the upper limit as <1.0, we essentially trade some risk of breakage for not having to bump the dependency every time a new version is uploaded, that is ok within our QA processes in Debian but almost certainly isn't appropriate upstream.