Simplify buildEnv: use `maps.Copy` for populating the map. (#546)

Co-authored-by: Mads Jensen <atombrella@users.noreply.github.com>

tfexec/cmd.go

diff --git a/tfexec/cmd.go b/tfexec/cmd.go
index acedc66..a5f17cb 100644
--- a/tfexec/cmd.go
+++ b/tfexec/cmd.go
@@ -12,6 +12,7 @@
 	"fmt"
 	"io"
 	"iter"
+	"maps"
 	"os"
 	"os/exec"
 	"runtime"
@@ -139,15 +140,11 @@
 		env = envMap(os.Environ())
 	} else {
 		env = make(map[string]string, len(tf.env))
-		for k, v := range tf.env {
-			env[k] = v
-		}
+		maps.Copy(env, tf.env)
 	}
 
 	// override env with any command specific environment
-	for k, v := range mergeEnv {
-		env[k] = v
-	}
+	maps.Copy(env, mergeEnv)
 
 	// always propagate CHECKPOINT_DISABLE env var unless it is
 	// explicitly overridden with tf.SetEnv or command env