go run command doesn't pick up non-go files in the same path

Folder structure:

- dev.env
- go.mod
- main.go

dev.env:

ENV="DEV"
PASSWORD="DEV!@#$%"

main.go:

package main

import (
	"fmt"
	"log"
	"os"
	"path/filepath"

	"github.com/joho/godotenv"
)

var environment string

func init() {
	fmt.Println("ENV: ", environment)
	dir, err := filepath.Abs(filepath.Dir(os.Args[0]))
	if err != nil {
		log.Fatal(err)
	}

	devEnvPath := filepath.Join(dir, "dev.env")
	_ = godotenv.Load(devEnvPath)
}

func main() {
	fmt.Println("PASSWORD", os.Getenv("PASSWORD"))
}

Command:

go run -ldflags="-X 'main.environment=DEV'" .                                                                        

Output:

ENV:  DEV
PASSWORD 

It works with go build but curious why it doesn't work with go run

As a general rule, don't use go run unless in the most trivial of use cases - it is the most common footgun in the Go community.

As @Marc pointed out the error stems from the go run binary is built in a temporary directory. To keep things simple, just use a relative path in your directory. This will work for both go build & go run:

dir := "."

devEnvPath := filepath.Join(dir, "dev.env")
err := godotenv.Load(devEnvPath)
if err != nil {
	log.Fatal(err)
}

As of Go 1.16, the simplest way to access files from the directory containing a package's source code is to use //go:embed and the embed package to embed those files into the compiled binary:

//go:embed dev.env
var devEnv string

func init() {
	m, err := godotenv.Unmarshal(devEnv)
	…
}