This exposes the `cargo pgrx` binary as a flake package called
`cargo-pgrx`. It can be built using `nix build .#cargo-pgrx`.

The systems specified in this flake were chosen from the project's
README of tested systems. The flake inputs were chosen to minimize the
maintenance burden.

Since the build runs tests in order to succeed, but the tests are
performed at build time for nix, the `test_parse_managed_postmasters`
test has been skipped. Since pgrx already has a test suite that runs
outside of nix, this should be a good compromise to allow the build to
work, but also keep the majority of the tests to ensure that nix users
are aware when they are broken. In addition to this, the `PGRX_HOME`
variable must be set for tests, so it is assigned to a tmpdir before
running the tests.

This script can be ran to update the rust-toolchain that is used by nix
builds. Since `fenix` is a flake input end-users are able to override it
themselves to whichever version they please, however it is useful to
have the provided version up to date, particularly if the project
ends up having an MSRV.

We could likely change this script to be one that updates all flake
inputs, however I find it useful to be able to update the rust
toolchain separately from the other dependencies and have chosen to
provide a script soley for that. This matches the existing `rustup.sh`
that is used to update the rustup toolchain version. If you wish to make
this file a generic `flake.lock` updater, remove `fenix` from the call
to `nix flake update` and it will update all inputs.

This function can be used by external nix flakes to build an extension
using pgrx. The caller must provide the postgres package, the rust
toolchain, extension source code, and flake system that they are
building for.

Most of this configuration has been converted from the previous nix
flake exposed from pgrx. I've made every attempt to trim away parts that
don't serve a purpose, but there's a chance that this could be
simplified slightly by someone who has more knowledge of pgrx than I do.

Co-authored-by: Sam Rose <samuel@supabase.io>