meta: add nix shell to build on Linux

As noted in [1] on Linux systems, it is not possible to build phoenixd due to a mismatch in the libc version.

A possible solution is to use Nix and build phoenixd inside the shell with all the dependencies. This way, the host machine can use the binary directly without downgrading libc, which can be dangerous.

[1] #1 (comment)
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

.nix/shell.nix

@@ -0,0 +1,13 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = [
+    pkgs.sqlite
+    pkgs.curl
+    pkgs.openjdk17
+
+    (pkgs.glibc.overrideAttrs (old: {
+      version = "2.19";
+    }))
+  ];
+}