[direnv] drop shell.nix, rely solely on devbox shell --print-env (#902)

## Summary

I don't think we need this shell.nix anymore.
The `devbox shell --print-env` should be providing the same information.

This should speed up the direnv time. 

## How was it tested?

the devbox project `devbox.json` has:
```
"env": {
  "PATH": "$PATH:$PWD/dist",
}
```

and after cd-ing into the directory for devbox repo, I see:
```
❯ echo $PATH | sed 's/:/\n/g' | grep dist
/Users/savil/code/jetpack/devbox/dist
```

we really need to add automated tests for these.

Author: savil

.envrc

@@ -3,9 +3,8 @@
 
 use_devbox() {
     watch_file devbox.json
-    if [ -f .devbox/gen/shell.nix ]; then
+    if [ -f .devbox/gen/flake/flake.nix ]; then
         DEVBOX_SHELL_ENABLED_BACKUP=$DEVBOX_SHELL_ENABLED
-        use nix .devbox/gen/shell.nix
         eval $(devbox shell --print-env)
         export DEVBOX_SHELL_ENABLED=$DEVBOX_SHELL_ENABLED_BACKUP
     fi

examples/data_science/jupyter/.envrc

@@ -3,9 +3,8 @@
 
 use_devbox() {
     watch_file devbox.json
-    if [ -f .devbox/gen/shell.nix ]; then
+    if [ -f .devbox/gen/flake/flake.nix ]; then
         DEVBOX_SHELL_ENABLED_BACKUP=$DEVBOX_SHELL_ENABLED
-        use nix .devbox/gen/shell.nix
         eval $(devbox shell --print-env)
         export DEVBOX_SHELL_ENABLED=$DEVBOX_SHELL_ENABLED_BACKUP
     fi

examples/development/ruby/.envrc

@@ -3,9 +3,8 @@
 
 use_devbox() {
     watch_file devbox.json
-    if [ -f .devbox/gen/shell.nix ]; then
+    if [ -f .devbox/gen/flake/flake.nix ]; then
         DEVBOX_SHELL_ENABLED_BACKUP=$DEVBOX_SHELL_ENABLED
-        use nix .devbox/gen/shell.nix
         eval $(devbox shell --print-env)
         export DEVBOX_SHELL_ENABLED=$DEVBOX_SHELL_ENABLED_BACKUP
     fi

examples/servers/nginx/.envrc

@@ -3,9 +3,8 @@
 
 use_devbox() {
     watch_file devbox.json
-    if [ -f .devbox/gen/shell.nix ]; then
+    if [ -f .devbox/gen/flake/flake.nix ]; then
         DEVBOX_SHELL_ENABLED_BACKUP=$DEVBOX_SHELL_ENABLED
-        use nix .devbox/gen/shell.nix
         eval $(devbox shell --print-env)
         export DEVBOX_SHELL_ENABLED=$DEVBOX_SHELL_ENABLED_BACKUP
     fi

examples/stacks/jekyll/.envrc

@@ -3,9 +3,8 @@
 
 use_devbox() {
     watch_file devbox.json
-    if [ -f .devbox/gen/shell.nix ]; then
+    if [ -f .devbox/gen/flake/flake.nix ]; then
         DEVBOX_SHELL_ENABLED_BACKUP=$DEVBOX_SHELL_ENABLED
-        use nix .devbox/gen/shell.nix
         eval $(devbox shell --print-env)
         export DEVBOX_SHELL_ENABLED=$DEVBOX_SHELL_ENABLED_BACKUP
     fi

internal/impl/generate.go

@@ -24,30 +24,20 @@ import (
 //go:embed tmpl/*
 var tmplFS embed.FS
 
-var shellFiles = []string{"shell.nix"}
-
 func (d *Devbox) generateShellFiles() error {
 
 	plan, err := d.ShellPlan()
 	if err != nil {
 		return err
 	}
 
-	outPath := filepath.Join(d.projectDir, ".devbox/gen")
-
-	for _, file := range shellFiles {
-		err := writeFromTemplate(outPath, plan, file)
-		if err != nil {
-			return errors.WithStack(err)
-		}
-	}
-
 	// Gitignore file is added to the .devbox directory
 	err = writeFromTemplate(filepath.Join(d.projectDir, ".devbox"), plan, ".gitignore")
 	if err != nil {
 		return errors.WithStack(err)
 	}
 
+	outPath := filepath.Join(d.projectDir, ".devbox/gen")
 	err = makeFlakeFile(outPath, plan)
 	if err != nil {
 		return errors.WithStack(err)

internal/impl/tmpl/envrc.tmpl

@@ -3,9 +3,8 @@
 
 use_devbox() {
     watch_file devbox.json
-    if [ -f .devbox/gen/shell.nix ]; then
+    if [ -f .devbox/gen/flake/flake.nix ]; then
         DEVBOX_SHELL_ENABLED_BACKUP=$DEVBOX_SHELL_ENABLED
-        use nix .devbox/gen/shell.nix
         eval $(devbox shell --print-env)
         export DEVBOX_SHELL_ENABLED=$DEVBOX_SHELL_ENABLED_BACKUP
     fi