ci: fix make build error

We only want to use the GOTOOLCHAIN if it's set otherwise fallback to
the go.mod toolchain version.

Error message:

    docker: invalid reference format.
    See 'docker run --help'.
    make: *** [Makefile:128: build-linux-arm-7] Error 125

See https://github.com/gotify/server/actions/runs/14690165536/job/41224267750

Author: jmattheis

Makefile

@@ -2,7 +2,7 @@ LICENSE_DIR=./licenses/
 BUILD_DIR=./build
 DOCKER_DIR=./docker/
 SHELL := /bin/bash
-ifndef GOTOOLCHAIN
+ifdef GOTOOLCHAIN
 	GO_VERSION=$(GOTOOLCHAIN)
 else
 	GO_VERSION=$(shell go mod edit -json | jq -r .Toolchain | sed -e 's/go//')