rpmbuild: Switch to centos-stream base image

For `make test-rpmbuild`, we currently are using registry.centos.org/centos
as the base image, but this has started causing build errors lately:

```
$ make test-rpmbuild
podman build -f images/rpmbuild/Containerfile .
STEP 1/6: FROM registry.centos.org/centos
STEP 2/6: WORKDIR $APP_ROOT/src
--> Using cache 7843ca52c49abdbf5bc2b56a55f32697442cc7251cdd69decce6108b26b0c546
--> 7843ca52c49
STEP 3/6: RUN yum -y install git-core rpm-build dnf-plugins-core 'dnf-command(builddep)'
CentOS Linux 8 - AppStream                       63  B/s |  38  B     00:00
Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist
Error: error building at STEP "RUN yum -y install git-core rpm-build dnf-plugins-core 'dnf-command(builddep)'": error while running runtime: exit status 1
```

This commit switches to quay.io/centos/centos:stream8 instead which does
not have such issues.

images/rpmbuild/Containerfile.in

@@ -1,4 +1,4 @@
-FROM registry.centos.org/centos
+FROM quay.io/centos/centos:stream8
 WORKDIR $APP_ROOT/src
 RUN yum -y install git-core rpm-build dnf-plugins-core 'dnf-command(builddep)'
 COPY . .