[UX] print statement for recomputing the nixEnv (#1521)

## Summary

This print-dev-env calculation may be a bit slow, so we should print a
statement
for the waiting user.

I do this in `ensurePackagesAreInstalled`, instead of in
`nix.PrintDevEnv` because otherwise we can print it 3x due to our cache
misses.

Note, this PR is focussed on the least-risky change for the release to
improve UX for improved wait times on switching to Remove Nixpkgs mode.

## How was it tested?

compiles

(cherry picked from commit e008f3b)

Author: savil

internal/impl/packages.go

@@ -234,6 +234,8 @@ func (d *Devbox) ensurePackagesAreInstalled(ctx context.Context, mode installMod
 	}
 
 	// Force print-dev-env cache to be recomputed.
+	// We may be able to remove this after improving cache hits.
+	fmt.Fprintf(d.stderr, "Recomputing the devbox environment.\n")
 	if _, err := d.computeNixEnv(ctx, false /*use cache*/); err != nil {
 		return err
 	}