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
x86_64 docker-alpine
lint

00:00:26

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

00:01:05

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

00:00:29

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

00:00:39

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

00:00:21

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

00:00:32

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

00:00:31

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

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