Fix relative paths with sops_decrypt_file()

sops_decrypt_file() computed the file path relative to the working
directory instead of relative to the Terragrunt configuration. This
generally worked with individual `terragrunt apply` operations but
failed for `terragrunt run-all apply`, as it would look for the file in
the parent directory.

Fixes #1319.

config/config_helpers.go

@@ -688,7 +688,7 @@ func sopsDecryptFile(params []string, trackInclude *TrackInclude, terragruntOpti
 	if err != nil {
 		return "", errors.WithStackTrace(err)
 	}
-	canonicalSourceFile, err := util.CanonicalPath(sourceFile, terragruntOptions.WorkingDir)
+	canonicalSourceFile, err := util.CanonicalPath(sourceFile, filepath.Dir(terragruntOptions.TerragruntConfigPath))
 	if err != nil {
 		return "", errors.WithStackTrace(err)
 	}
@@ -697,7 +697,7 @@ func sopsDecryptFile(params []string, trackInclude *TrackInclude, terragruntOpti
 		return val, nil
 	}
 
-	rawData, err := decrypt.File(sourceFile, format)
+	rawData, err := decrypt.File(canonicalSourceFile, format)
 	if err != nil {
 		return "", errors.WithStackTrace(err)
 	}