build: do not add testing repo for stable branches

The testing repo is only available for edge, so do not add it to the
repositories file when building for stable releases.

Fixes: alpine/infra/infra#10670
9 jobs for no-testing-for-stable-branches in 2 minutes and 38 seconds
Status Job ID Name Coverage
  Verify
passed #44433
docker-alpine x86_64
lint

00:00:26

 
  Build
passed #44438
docker-alpine ci-docker-image aarch64
build-aarch64

00:01:05

passed #44439
docker-alpine ci-docker-image armv7
build-armv7

00:00:29

passed #44437
docker-alpine ci-docker-image ppc64le
build-ppc64le

00:00:39

passed #44434
docker-alpine ci-docker-image s390x
build-s390x

00:00:21

passed #44518
x86 ci-docker-image docker-alpine
build-x86

00:00:32

passed #44517
x86_64 ci-docker-image docker-alpine
build-x86_64

00:00:31

failed #44436
docker-alpine ci-docker-image x86
build-x86

failed #44435
docker-alpine ci-docker-image x86_64
build-x86_64