[print-dev-env] Skip cache if lockfile has changed (#1674)

## Summary

We need to skip the PrintDevEnvCache in ComputeNixEnv if lockfile is
stale.

`ensurePackagesAreInstalledAndComputeEnv` had made an assumption that
`ensurePackagesAreInstalled` will have called `computeNixEnv` already to
ensure
the PrintDevEnvCache was updated. However, after some recent
refactoring, we
no longer call `computeNixEnv` within `ensurePackagesAreInstalled`.

So, we now need to check the lockfile status, and skip the
PrintDevEnvCache
if lockfile is stale.

## How was it tested?

1. Had `patch_glibc:true` for `hello` package. Did `devbox shell` and
then
saw that `which hello` points to the patch-glibc flake's hello.

2. exited devbox shell, and edited devbox.json to `patch_glibc:false`

3. re-started `devbox shell` and `which hello` now shows the regular nix
`hello`.
Previously, it would still show the patch-glibc hello since `PATH` would
not have
been updated to omit that.

Author: savil

internal/impl/devbox.go

@@ -969,7 +969,7 @@ func (d *Devbox) ensurePackagesAreInstalledAndComputeEnv(
 	// it's ok to use usePrintDevEnvCache=true here always. This does end up
 	// doing some non-nix work twice if lockfile is not up to date.
 	// TODO: Improve this to avoid extra work.
-	return d.computeNixEnv(ctx, true)
+	return d.computeNixEnv(ctx, true /*usePrintDevEnvCache*/)
 }
 
 func (d *Devbox) nixPrintDevEnvCachePath() string {

internal/impl/packages.go

@@ -289,6 +289,13 @@ func (d *Devbox) ensurePackagesAreInstalled(ctx context.Context, mode installMod
 		return err
 	}
 
+	// Use the printDevEnvCache if we are adding or removing or updating any package,
+	// AND we are not in the shellenv-enabled environment of the current devbox-project.
+	usePrintDevEnvCache := mode != ensure && !d.IsEnvEnabled()
+	if _, err := d.computeNixEnv(ctx, usePrintDevEnvCache); err != nil {
+		return err
+	}
+
 	// Ensure we clean out packages that are no longer needed.
 	d.lockfile.Tidy()