[nipkgs version] add feature flag and write to devbox.json if missing

[savil]

Summary

We add nixpkgs.version to devbox.json if it is missing.

How was it tested?

> cd testdata/rust/rust-stable

# control
> DEVBOX_DEBUG=1 devbox shell
> git diff devbox.json
# no change as expected


> DEVBOX_FEATURE_NIXPKG_VERSION=1 DEVBOX_DEBUG=1 devbox shell
> git diff devbox.json
diff --git a/testdata/rust/rust-stable/devbox.json b/testdata/rust/rust-stable/devbox.json
index 89b7c0d..028385d 100644
--- a/testdata/rust/rust-stable/devbox.json
+++ b/testdata/rust/rust-stable/devbox.json
@@ -7,5 +7,8 @@
     "init_hook": [
       "source init-shell.sh"
     ]
+  },
+  "nixpkgs": {
+    "version": "af9e00071d0971eb292fd5abef334e66eda3cb69"
   }
-}
+}
\ No newline at end of file

[savil]

Current dependencies on/for this PR:

• main
  • PR [nipkgs version] add feature flag and write to devbox.json if missing #252 👈

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