Fix #404: nil map error when reading env file (#405)

Co-authored-by: Mark Glines <mark@glines.org>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/405
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: infinoid <infinoid@noreply.gitea.com>
Co-committed-by: infinoid <infinoid@noreply.gitea.com>
(cherry picked from commit 934471813a41efabf6a9b5d5b22a2a5770ff12ce)

internal/pkg/config/config.go

@@ -99,6 +99,9 @@ func LoadDefault(file string) (*Config, error) {
 			if err != nil {
 				return nil, fmt.Errorf("read env file %q: %w", cfg.Runner.EnvFile, err)
 			}
+			if cfg.Runner.Envs == nil {
+				cfg.Runner.Envs = map[string]string{}
+			}
 			for k, v := range envs {
 				cfg.Runner.Envs[k] = v
 			}