nix: added flake and derivation for the project

Signed-off-by: markoburcul <marko@status.im>

.gitignore

@@ -5,7 +5,11 @@ target/
 
 # Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
 # More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
-Cargo.lock
+# Commented since for nix derivation we need the Cargo.lock file.
+# Cargo.lock
+
+# If built with Nix, the target with be within result directory.
+result/
 
 # These are backup files generated by rustfmt
 **/*.rs.bk