Read the rclone config file named by RCLONE_CONFIG (closes #321)

An rclone destination whose remote is defined only in the file
RCLONE_CONFIG names was reported as not found: rclone reads that
variable only as the value of its --config flag, which vaultik does not
have, so vaultik always used rclone's default config file.
NewRcloneStorer now sets rclone's config path from RCLONE_CONFIG when
the variable is set, as that flag would, including an empty value,
which rclone takes to mean no config file.

Model: opus-5-5

internal/storage/rclone.go

@@ -7,10 +7,12 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"os"
 	"strings"
 	"time"
 
 	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/config"
 	"github.com/rclone/rclone/fs/config/configfile"
 	"github.com/rclone/rclone/fs/operations"
 
@@ -33,6 +35,15 @@ type RcloneStorer struct {
 // The remote parameter is the rclone remote name (as configured via `rclone config`).
 // The path parameter is the path within the remote.
 func NewRcloneStorer(ctx context.Context, remote, path string) (*RcloneStorer, error) {
+	// rclone reads RCLONE_CONFIG only as the value of its --config flag,
+	// which vaultik does not have, so it is applied here as that flag would.
+	if configPath, ok := os.LookupEnv("RCLONE_CONFIG"); ok {
+		err := config.SetConfigPath(configPath)
+		if err != nil {
+			return nil, fmt.Errorf("using RCLONE_CONFIG %q: %w", configPath, err)
+		}
+	}
+
 	// Install the default config file handler
 	configfile.Install()