Dropping el8stream builds

Since CentOS Stream 8 is now EOL and repos moved to vault the el8 build no longer works. Dropping it from configuration, but keeping the matrix structure so that it can be reintroduced (e.g. on stable RHEL clones or whatnot)

.github/workflows/build.yml

@@ -11,10 +11,6 @@ on:
       keep_workdir:
         description: keeps the working dir on the runner after the job is finished
         required: false
-      exclude-distro-num:
-        description: elXstream distro NOT to run. Use 8 or 9.
-        required: true
-        default: "8"
       distro-variant:
         description: Build with CBS or COPR packages. Use "-cbs" or "-copr".
         required: true
@@ -31,9 +27,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        num: [8, 9]
-        exclude:
-          - num: ${{ github.event.inputs.exclude-distro-num }}
+        num: [9]
     name: build-el${{ matrix.num }}stream${{ github.event.inputs.distro-variant == '' && '-copr' || github.event.inputs.distro-variant }}
     runs-on: [image-builders, "el${{ matrix.num }}"]
     container: