ci: don't build on arm64

The build fails with the following error:

    	#25 [linux/arm64 11/11] RUN ./gradlew tasks
	#25 45.75 "Install Android SDK Platform 28 (revision: 6)" ready.
	#25 45.75 Installing Android SDK Platform 28 in
	    /app/android-sdk/platforms/android-28
	#25 45.75 "Install Android SDK Platform 28 (revision: 6)" complete.
	#25 45.75 "Install Android SDK Platform 28 (revision: 6)" finished.
	#25 45.85
	#25 45.85 FAILURE: Build failed with an exception.
	#25 45.85
	#25 45.85 * What went wrong:
	#25 45.85 A problem occurred configuring project ':OpenEdXMobile'.
	#25 45.85 > Failed to install the following SDK components:
	#25 45.85       build-tools;28.0.3 Android SDK Build-Tools 28.0.3
	#25 45.85   Install the missing components using the SDK manager in
	    Android Studio.

This issue is tracked in #7.

.gitlab-ci.yml

@@ -4,6 +4,7 @@ variables:
     TUTOR_PYPI_PACKAGE: tutor-android
     OPENEDX_RELEASE: olive
     GITHUB_REPO: overhangio/tutor-android
+    IMAGES_BUILD_PLATFORM: "linux/amd64"
 
 include:
   - project: 'community/tutor-ci'