Remove go mod tidy from Docker build

go mod tidy in Docker build causes disk space issues on CI runners
(compiling modernc.org/ccgo pulls many C files). go.mod/go.sum are
committed to repo, so go mod download + go build is sufficient.

Dockerfile

@@ -13,8 +13,8 @@ RUN go mod download
 # Copy source
 COPY . .
 
-# Tidy and build binary
+# Build binary
-RUN go mod tidy && CGO_ENABLED=0 GOOS=linux go build -o /ym-dl ./cmd/ym-dl
+RUN CGO_ENABLED=0 GOOS=linux go build -o /ym-dl ./cmd/ym-dl
 
 # Runtime stage
 FROM gcr.io/distroless/static-debian12:nonroot