[env] Fixed env_from bug when it is called from a subdirectory (#2355)

## Summary
Fixes this bug:
https://github.com/jetify-com/devbox/pull/2174/files#r1659165116

## How was it tested?
- use compiled devbox
- put `export FOO = 'BBBAR'` in .env file
- put the following in a devbox.json file
```json
  {
    "packages": [],
    "shell": {
      "init_hook": [
        "echo 'Welcome to devbox!' > /dev/null"
      ],
      "scripts": {
        "test": [
          "echo $FOO"
        ]
      }
    },
    "env_from": ".env"
  }
```
- create a subdirectory besides devbox.json called `test`
- cd test && devbox run test

internal/devconfig/configfile/env.go

@@ -25,10 +25,13 @@ func (c *ConfigFile) ParseEnvsFromDotEnv() (map[string]string, error) {
 	if !c.IsdotEnvEnabled() {
 		return nil, fmt.Errorf("env file does not have a .env extension")
 	}
-
-	file, err := os.Open(c.EnvFrom)
+	envFileAbsPath := c.EnvFrom
+	if !filepath.IsAbs(c.EnvFrom) {
+		envFileAbsPath = filepath.Join(filepath.Dir(c.AbsRootPath), c.EnvFrom)
+	}
+	file, err := os.Open(envFileAbsPath)
 	if err != nil {
-		return nil, fmt.Errorf("failed to open file: %s", c.EnvFrom)
+		return nil, fmt.Errorf("failed to open file: %s", envFileAbsPath)
 	}
 	defer file.Close()