[offline] Allow devbox to work offline if DNS fails (#1985)

## Summary

Fixes #1977

(with another hack).

Unfortunately the way we structure our flake makes it really difficult
to remove our network requests (when state is stale). Ideally, the flake
can be built off of local only nix store references, but that's a much
bigger project.

In the meantime, this is a quick hack that fixes issue above.

## How was it tested?

Added synthetic error that returns `connection refused` in the message.
Ensures I was still able to run/shell with a stale state.

internal/devbox/devbox.go

@@ -1007,10 +1007,21 @@ func (d *Devbox) ensureStateIsUpToDateAndComputeEnv(
 
 	// When ensureStateIsUpToDate is called with ensure=true, it always
 	// returns early if the lockfile is up to date. So we don't need to check here
-	if err := d.ensureStateIsUpToDate(ctx, ensure); err != nil && !strings.Contains(err.Error(), "no such host") {
-		return nil, err
+	if err := d.ensureStateIsUpToDate(ctx, ensure); isConnectionError(err) {
+		if !fileutil.Exists(d.nixPrintDevEnvCachePath()) {
+			ux.Ferror(
+				d.stderr,
+				"Error connecting to the internet and no cached environment found. Aborting.\n",
+			)
+			return nil, err
+		}
+		ux.Fwarning(
+			d.stderr,
+			"Error connecting to the internet. Will attempt to use cached environment.\n",
+		)
 	} else if err != nil {
-		ux.Fwarning(d.stderr, "Error connecting to the internet. Will attempt to use cached environment.\n")
+		// Some other non connection error, just return it.
+		return nil, err
 	}
 
 	// Since ensureStateIsUpToDate calls computeEnv when not up do date,

internal/devbox/errors.go

@@ -0,0 +1,12 @@
+package devbox
+
+import "strings"
+
+func isConnectionError(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	return strings.Contains(err.Error(), "no such host") ||
+		strings.Contains(err.Error(), "connection refused")
+}